Friday, February 23, 2018

Borehole Stability

Rock mechanics theory and practice are being stretched to their limit to solve severe borehole stability problems in the tectonically active Cusiana field of Colombia. The earth scientists' and drilling engineers' main challenge is estimating those most elusive of all earth parameters, subsurface stress and rock strength. What they find out can influence the entire development strategy of this newly discovered, giant field.

 Drilling and maintaining in-gauge hole remains one of the driller's greatest challenges. In-gauge, stable hole not only means trouble-free drilling, it also helps ensure that logs are of high quality, that the cement job runs smoother, and that every subsequent action in the well occurs to the operator's maximum advantage. Everything in the oilfield starts with the drillhole, so ensuring the hole possible is worth some thought and expense.

Fighting for hole integrity, the driller must monitor and juggle two key factors- mud chemistry and mud weight. Mud and formation must be balanced chemically, particulary in shales, to prevent the formation swelling against the drillpipe or sloughing into borehole. Simultaneously, a mechanical balance must be achieved to prevent two well-known phenomena - breakouts and formation fracturing -and ,as experts now suspect, maybe also a third phenomenon called shear displacement.  This article focueses on how the mechanical balance is monitored and achieved, and reviews the latest theories and measurement techniques in a case study from the newly discovered Cusiana field in eastern Colombia.

Rock in its natural state is stressed in three principal directions - vertically from the overburden, Sv, horizontally in two orthogonal directions. The two horizontal stresses are generally not equal - the maximum and minimum horizontal stresses are expressed as SH and Sh, respectively.

As a borehole is driled, hydraulic pressure of the drilling mud must replace the support lost by removal of the original column of rock. But mude pressure being uniform in all directions cannot exactly balance the earth stress. Consequently, rock surrounding the borehole is distorted or strained, and may fail if the redistributed stresses exceed rock strength.

One failure mechanism is tensile failure. This occurs when hydraulic pressure of the mud becomes too high, causing formation stress at the borehole wall to go into tension  and exceed the rock's tensile strength. This fractures the rock along a plane perpendicular to the direction of the earth's minimum stress, generally one of the horizontal stresses, and potentially causes lost circulation.

Alternatievely, the formation can fail in compression. This is commonly predicted using the Mohr-Coulomb model, in which the factors controlling failure are the minimum and maximum stresses at the borehole wall- the intermediate stress is assumed to have no effect. Compressive failure may be caused by too low or too high a mud weight. In either case, formation caves in or spalls off, creating breakouts. The debris can then accumulate in the borehole leading to stuck pipe or even hole collapse.

The third, recently discovered mechanism of mechanical instability is shear displacement. In this, mud pressure is high enough to reopen naturally existing fractures that intersect the borehole. As the fracture is opened, stresses along it are temporarily relieved, and opposite faces of the fracture can physically shear, creating a small but potentially dangerous dislocation in the borehole. The phenomenon was first identified by Elf geologist Maury and Sauzy in agas field in the southeast France.

 The bottom line for the driller striving to maintain a stable hole is choosing the right mud weight. First, mud weight must be sufficient to exceed formation pore pressure, not a mechanical stability issue but essential for preventing kicks. Second, it must be high enough to avoid the low mud-weight mode of compressional failure. Third, it must not be too high that the formation fails in tension or in the high mud-weight mode of compressional failure. Fourth, it must not be too high to initiate shear displacement.

Steering a middle course would be challenging enough in a thick, homogeneous bed, but in reality drillers must maintain stability over long openhole sections with varying lithology, strengths and stresses. And generally speaking, the middle course is harder to find the more the well is deviated. When the going gets too rough, casing gets set. But while drilling continues, all available measurements and knowledge, particualry experience drilling nearby wells, must be applied to choose optimum mud weight.

The key parameters for evaluating stability are simply the three principal components of earth stress and rock strength parameters defining tensile and compressional failure. Once these are known, computer programs seedily calculate the principal stresses at the borehole wall, and predictions of failure can be obtained. But both earth stresses and rock failure are extraordinarily difficult to assess accurately, and a successful resolution must use an integration of all available methods of obtaining them. Most of these methods have been brought to bear with some success in the Cusiana field where drillers have faced immense drilling problems.

The Cusiana field lies in the eastern foothills of the Andes. Discovered in 1991 by BP exploration company (Colombia) Ltd. with partners Total Compagnie Francaise des Petroles and Triton Energy Corp., the field promises to be in the top 50 oil reserves of the world. Drilling the discovery wells, though, proved a driller's nightmare with mechanical stability problems causing stuck pipe, damaged casing and sidetracks. BP estimates that approximately 10% of well cost is spent coping with bad hole. Average rig time to reach oil-bearing sediments at more than 14,000 ft [4200 m] has been 10 months. 

Although both chemical and mechanical factors inevitably contribute to bad hole in the Cusiana field, the operators had no doubt that mechanical stability was the prime cause. The field lies on the eastern flank of the Oriental Andes cordillera, an area being actively compressed by the eastward thrust of the Pacific "Nazca" plate and the more southeasterly thrust of the Carribean plate. 

Three fault blocks are being compressed in the Cusiana region with faults striking parallel to the cordillera - the Yopal block under the cordillera, the Cusiana block containing the reservoir, and the Llanos block under the plains to the southeast. The reservoir is situated in the Cusiana block between the Yopal and Cusiana thrust faults.

Generally, in quiescent geological areas, vertical stress caused by the weight of the overburden is greater than either of the two horizontal stresses. Geologists suspected this might be the case in the Yopal block, where there seemed to be enough minor faulting to relieve the large horizontal stress induced by tectonic deformation. But in the Cusiana block and particularly the Llanos block, it was thought that horizontal stress in the northwest-southeast direction was still high, almost certainly greater than the vertical stress. 

 Drillers became convinced of this as severe drilling problems developed in the Carbonera formations in the middle of the Tertiary zone. The Carbonera formation is an interbedded sequence of sands, mudstones and shales comprising eight units. As a result, a task force comprising BP, Total, Triton and Schlumberger experts was convened to measure and understand the Cusiana stress field and attempt to resolve the drilling problems.

The techniques that are being used to evaluate the Cusiana stress field are multifarious. Some remain to be implemented, others already have been. They include: 
  • computer simulation of the region's tectonic deformation.
  • Analysis of the breakouts on caliper logs to determine the direction of minimum horizontal stress.
  • Analysis of oriented cores to evaluate stress direction and possibly magnitude.
  • Use of extended leakoff test to evaluate horizontal stress magnitudes and calibrate an earth stress model.

BP's computer simulation of tectonic deformation in the three blocks confirmed that in most of the region, horizontal stress in the direction of tectonic compression -that is, in a northwest-southeast direction, normal to the cordillera -was the maximum stress. With no compression, vertical stress was greater than the normal horizontal stress. But with some compression, normal horizontal stress increased, eventually becoming larger than the vertical stress. 

In the field, the direction of least horizontal stress can be confirmed by observing breakouts using caliper logs. Breakouts, caused by the borehole being in compressional failure, have been observed worldwide to cause ovalization of the borehole with oval's long axis parallel to the minimum stress. In several Cusiana wells, breakouts have been evaluated using the dual caliper and borehole drift measurements of dipmeter and resistivity imaging tools. The caliper pairs, oriented at 90 degree , provide information about borehole enlargement. The tools' navigation system measures orientation and deviation of the borehole and establishes tool azimuth.

Breakouts can be picked automatically from logs using the Breakout Orientation Log (BOL) program. This reviews the caliper pairs and flags zones where one pair is close to bit size while the other is significantly larger. Furthermore, their difference has to materialize quickly versus depth to distinguish potential breakouts from irregular washout. Once flagged, the long axis of the breakout can be oriented with the borehole drift and tool azimuth measurements. The results in the Cusiana field have a striking consistency, showing breakout and therefore direction of minimum horizontal stress oriented between 30 degree and 60 degree that is, southwest-northeast.

A more common and certainly more reliable method of measuring earth stress is through extended leakoff, or minifrac tests. Normal leakoff tests are systematically performed by drillers to investigate rock strength after casing is set. The hole is drilled out several feet below casing and borehole fluid pressure is increased by pumping small quantities of additional mud into the well. At first, the pressure builds up linearly. But when formation at the borehole wall cracks and mud begins to leak off, presssure increases less fast. The point where this change occurs gives the leakoff pressure. In the Cusiana field, BP has so far performed 35 leakoff test in 11 wells.

The extended leakoff test is a riskier exercise for the driller and one not contemplated during the trials of drilling the current Cusiana wells. As before, mud is pumped until intial failure of the formation at the borehole wall. But then more mud is pumped to create full fracture - this occurs at the slighty higher breakdown pressure, Pbd. As the fracture forms, pressure usually drops. After the fracture is extended for a while, pumping is stopped, and pressure is carefully monitored as the fracture closes. At the very moment of closure, the pressure levels of slightly. Pressure at the point exactly equals the minimum horizontal stress, Sh. In a second cycle, mud can be pumped again to reopen the fracture. The difference between Pbd and the pressure required to reopen gives the tensile strength of the formation, T0.

Basic borehole stability theory shows that in the case of initiating a vertical fracture, leakoff pressure, Plo , is

Plo= 3Sh-Sh + To- Pp'

where Pp is formation pore pressure, measurable using RFT Repeat Formation Tester or MDT Modular Formation Dynamics Tester tools. 


At 8500 ft [2590 m] in one of the Cusiana wells, a leakoff pressure of 14.4 lbm/gal was recorded -effective mud weight is used here instead of absolute pressure. Pore pressure was known to be equivalent to 8.96 lbm/gal and the formation was assessed as being very weak so T0 was ignored. The above equation then gives that Sh equals 19.84 lbm/gal. Integrating the density log in the well provided a vertical stress of 20.2 lbm/gal, so it appears in this case that Sh = Sv> Sh.

Stress estimates from leakoff data are obtained at sporadic depths only, so the next step is interpolating them to obtain a continous estimate versus depth. This is achieved in two steps. First, the stress estimates are used to calibrate a simple earth model that relates horizontal stresses to the vertical stress. Second, with log data as input, the model is used to evaluate the horizontal stresses foot by foot down to reservoir depth.

Several models are available. One assumes the earth behaves elastically, squeezed vertically by the overburden and laterally by tectonic forces ; another assumes that as overburden increases, the earth continually fails according to the Mohr-Coulomb criterion. In all these models, horizontal and vertical stresses are related via the formation's elastic constants, and these are derivable only from wireline logs of density and compressional and shear acoustic velocity.

Density and compressional acoustic velocity have long been standard wireline measurements, but only recently has a shear velocity logging measurement been feasible in all types of formation - previously it could be measured in hard formation, but not in soft formation such as shales, mudstones and sands.

Conventional sonic logging tools employ a monopole energy source that produces an omnidirectional pressure pulse in the mud that excites both compressional and shear waves in the formation. But the shear waves are detectable in the borehole only if their velocity is faster than the acoustic velocity of the mud, not the case when logging soft formations. In the new DSI Dipole Shear Sonic Imager tool, a dipole source displaces the borehole horizontally to create both shear and flexural waves in the formation. Together, these provide a measurable shear velocity in any formation type.

After earth stress versus depth is evaluated, the next step toward assessing safe mud weight is estimating rock strength, in both compression and tension. This can be achieved in the laboratory with destructive testing of core samples, or, in the absence of cores, using correlations from logging measurements. Several correlations are used. In one, compressive strenth is made a function of porosity, in another it is a function of the rock's shear modulus. In all cases, the same three logging measurements - density, compressional acoustic velocity and shear acoustic velocity are usually required. 

 A borehole stability log over 4000 ft [1200 m] of troublesome formations in one of the Cusiana wells is shown with highly compressed vertical scale. The caliper log in track 1 shows severe washouts in the top half of the section and slightly better hole condition in the lower half. Track 2 shows three elastic parameters calculated from density and acoustic measurements. The high Poisson ratio of 0.30 and relatively low Young modulus of 3x10^6 psi indicate that all formations are fairly weak. 


Friday, February 2, 2018

Structural Interpretation in Offshore Congo

Petroleum exploitation begins with geologist and geophysicst exploring unknown territory with few wells for guidance and no visible sign of oil and gas. Their task is to uncover the best prospects and decide whether oil companies should commit to millions of exploration dollars. A key tool is structural interpretation of seismic data. A survey shot in deep waters off the Congo mainland shows how geologist apply this technique. 

 Play in exploration context describes a geological configuration that favors the accumulation of hydrocarbons. Plays and their associated prospects are what exploration geologist sped most of their professional lives looking for.

 Until oil or gas is discovered, plays exist mostly in the mind. Focusing on one or another of the earth's basins, explorationists search for that seemingly impossible concatenation- source rock, migration path, reservoir and seal. Each vital ingredient must be present, both at the correct physical location and at the right time.

 Source rocks rich enough in organic material must have been buried and heated to sufficiently high temperature and long enough to form petroleum. Petroleum migrates upward, so there must have been a conduit to guide it. And a porous, permeable reservoir rock capped by impermeable rocks is required to receive and trap the fluid. Finally, the ensuing geological evolution - commonly several tens or even hundreds of million years - must have left the reservoir and seal intact. 

As explorationists focus on a sedimentary basin, this juxtaposition of geological coincidence occupies the mind. Every scrap of evidence is used to refine the notion of how the basin might have evolved and whether the structural and sedimentary history might favor a play. Outcrop geology, satellite imagery, magnetic and gravity surveys, and especially seismic data contribute to the interpretation process. This article shows how various plays in offshore Congo crystalized in the minds of geologists as they reviewed a nonproprietary seismic survey shot in West African waters.

Geologist first became interested in the Congo in 1928, attracted by tar seepages near Pointe Noire on the coast and knowledge that the subsurface contained thick deposits of salt- salt intrusions provide a classic trapping mechanisms for hydrocarbon accumulation. The salts had been identified in numerous boreholes drilled for potash mining exploration.

Oil was discovered at Pointe Indienne, 20 kilometers north of Pointe Noire, in 1959. But further onshore exploration proved fruitless - some oil shows , but nothing of commercial value. In the late 1960s, an Elf/AGIP partnership discovered oil offshore. Further successes yielded five offshore fields and oil production  from the Congo now exceeds 115,000 barrels per day. Recoverable reserves are currently estimated at more than a billion barrels. 

The GECO_PRAKLA nonproprietary survey of 1990, which forms the basis of the interpretation described in this article, covered 15,000 square kilometers. The survey ventured from shallow water at the edge of the continental shelf, where oil plays were known to exist, to depths up to 2000 m [6560 ft]. This is deeper than the current limit of commercial exploitation, but the deepwater data were nevertheless crucial for elucidating basin structure and migration pathways from deep in the basin. The survey lines were designed to connect with previous surveys conducted closer to the coast and intersect any exploration wells where precise lithological data would be available.

The offshore basins of the Congo form part of the West African Salt Basin, a large collection of basins stretching 2000 km from south Cameroon to Angola. The salt was deposited during Aptian times about 120 million years ago when the rifting of South America from West Africa gradually evolved into a full-fledged drift. Later salt movement would create anticlinal folds capable of trapping hydrocarbons. 

The story , begins thirtly million years earlier during the Late Jurassic. At this time, extensional faulting and subsidence took place in the part of the Gondwana supercontinent that would eventually become both the east coast of South America and the west coast of Africa. Further stretching or extension in the Early Cretaceous led to the formation of a large-scale rift along the future western Africa and eastern Brazilian margins. A modern parallel is the Great Rift Valley extending from the Red Sea to the Zambesi River.

Initially, the rift and its basins were above sea level and isolated from the ocean. Large lakes formed in which sandstone and shales accumulated. Some shales were deposited in oxygen-deficient water, allowing preservation of organic matter. These formations are the source for billions of barrels of oil found in the West African basin. 

By Aptian times, continued subsidence and a rise in global sea level permitted incursion by the sea. At first, this was intermittent, with the sea alternately entering and receding from the basins. This created ideal conditions of repeated evaporation and marine flooding to create thick deposits of halite, the Aptian salts.

Later, during the mid to Late Cretaceous, the area was definitely submerged and continental breakup of Gondwana led to a separation, or drift, of South America from Africa. Whreas the basins had previously been linked on one continental plate, now they were separated by a widening tract of ocean, as the Atlantic opened through injection of new oceanic crust at the mid-ocean ridge. Sedimentation was now marine, with thick deposits of limestone, sandstone and shale. Further subsidence took place in the Late Tertiary and was probably associated with faulting related to the collision of the Eurasian and African plates.

This big picture the interpreters learned from extensive research into the region's geological history. Another card in their hand was knowing which formations had produced oil shows during drilling in the area , which of these had produced commercially, and which formation was the most likely source for the shows - the most important was the Marnes Noires, or black marls, continental deposits formed under lacustrine conditions late in the rift phase and prior to the invasion of sea water. 

The majority of commercial reservoirs in offshore Congo occurs in sandy and dolomitic rock deposited during the early drift phase in the mid-Cretaceous, with structural traps created by movement of the transition-phase Aptian salts. Noncommercial oil shows have been found aplenty in rift and transition rocks of the Lower Cretaceous , but no oil has been tested in Upper Cretaceous or Tertiary formations.

The interpreters knew the source of oil, therefore, and in general how it most  likely migrated and became trapped. What they did not know -and what they sought during the interpretation - was information on the three-dimensional structure of the deepwater Congo sediments -specifically the location, general distribution and size of likely hydrocarbon prospects. Their immediate goal was to identify target areas suitable for detailed mapping with more closely spaced surveys. 

We rejoin our interpreters as they inspect the survey's processed and migrated sections and begin the critical task of identifying formation tops - the lenghty data processing was previously accomplished.


 Scanning the sections, the interpreter will also note major structural features such as listric faulting and salt bodies, and begin the highly skilled task of constructing a picture of the subsurface, all the time drawing and updating conclusions on a working map. Perhaps the interpreter's most cherished skill is this ability to visualize in three dimensions.

The basis of structural seismic interpetation is the loop method, in which a seismic reflector representing a geologic horizon is mapped around a series of intersecting sections and then back to original section.Closing the loop is easier said than done. Tracing the continuity of a reflector can be tricky across a fault and sometimes impossible if the formation pinches out laterally or has been eroded to form an unconformity. Problems may also occur in areas of steep dip, where 2D migration on each section fails to image complex three-dimensional structure correctly, producing a slight mistie.

Typically, the interpreter plots the major geological units from the logs on the seismic section at the well location to obtain the best correlation. Lithology changes observed on the logs assist the correlation process. For example, a slow formation such as mudstone overlying a fast formation such as tigh limestone typically correlates with a white band or trough on the seismic section. Conversely, a fast formation overlying a slow formation generally correlates with a black band or peak.  

Once the best correlation is found, the interpreter colors the section at the well location according to the geological units on the log.

Some interpreters identify all faults on the section (red marking) and then track reflector continuity. Others may concentrate on reflectors and reflector terminations- onlaps and truncations - concentrating on the stratigraphy and marking in only those faults that bear on the work at hand.

In both example sections, the thin Aptian sand (orange) forms a boundary between the rift sediments below and the drift sediments above. Overlying the sandstone, the salt (purple) intrudes into the overlying sediments, creating structural traps for oil generated in the Marnes Noire below.

The first section that lies parallel to the direction of geological dip cleary shows fault blocks in the deep rift sediments and large-scale listric faults in the shallower drift sequences. A listric fault has a pronounced curved slip face. In this example, the sediments to the left (southwest) of the fault have been displaced downward and rotated clockwise. 

The interpretation seems clean and finished, yet questions often remain. Formation tops may be uncertain, particulary in the deeper section beyond the range of well control and where seismic data lose their resolution. The exact shape of the all important salt intrusions may be subject to different interpretations. A solution for resolving these cases lies with magnetic and gravity data, acquired concurently with the seismic data. 

At this point, all the factors necessary to pinpoint plays are at hand. The burial history confirms the potential of organic-rich horizons as source. Detailed comprehensive and mapping of basin structure reveals likely migratory paths and trapping mechanisms. The thickness maps indicate the distribution and geometery of sediment bodies and assists in recognizing commercially interesting reservoirs. Deciding the location of plays now demands of the interpreter a juggling of these factors and the picking of locations and depths where all indications appear simultaneously favorable. The result is a play map that oil companies can use to decide