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. 


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