Sunday, May 13, 2018

The State of the Water Base Mud Art

This article will now concentrate on advances in water base mud (WBM) technology by looking at two distinct directions of development: the use of polyols for shale inhibition and the introduction of mixed-metal hydroxides to improve hole cleaning and help reduce formation damage.

Polyol muds-Polyol is the generic name for a wide class of chemicals-including glycerol, polyglicerol or gylcols such as proplylene glycol-that are usually used in conjunction with an encapsulating polymer (PHPA) and inhibitive brine phase (KCl). These materials are nontoxic and pass the environmental protocols, including those laid down in Norway, the UK, The Netherlands, Denmark and the USA.

 Glycols in mud were proposed as lubricants and shale inhibitors as early as the 1960s. But it was not until the late 1980s that the materials became widely considered. Properly engineered polyol muds are robust, highly inhibitive and often cost-effective. Compared with other WBM systems, low volumes are typically required. Polyols have a number of different effects, such as lubricating the drillstring, opoosing bit balling (where clays adhere to the bit) and improving fluid loss. Today, it is their shale-inhibiting properties that attract most attention. For example, test carried out by BP show that the addition of 3 to 5% by volume of polyglycol to a KCl-PHPA mud dramatically improves shale stabilization. 

Field experience using polyol muds has shown improved wellbore stability and yielded cuttings that are harder and drier than those usually associated with WBM.  

Mixed-metal hydroxide (MMH) mud - MMH mud has a low environmental impact and has been used extensively around the world in many situations: horizontal and short-radius wells, unconsolidated or depleted sandstone, high-temperature, unstable shales, and wells with severe lost circulation. Its principal benefit is excellent hole-cleaning properties.

Many new mud systems-including polyol muds -are extensions of existing fluids, with perhaps a few improved chemicals  added. However, MMH mud is a complete departure from existing technology. It is based on an insoluble inorganic, cyrstalline compound containing two or more metals in a hydroxide lattice-usually mixed alumunium/magnesium hydroxide, which is oxygen-deficient. When added to prehydrated bentonite, the positively charged MM particles interact with the negatively charged clays forming a strong complex that behaves like an elastic solid when at rest. 

This gives the fluid its unusual rheology: an exceptionally low plastic viscosity-yield point ratio. Conventional muds with high gel strength usually require high energy to initiate circulation, generating pressure surges in the annulus once flow has been established. Although MM has great gel strength at rest , the structure is easily broken. So it can be transformed into a low-viscosity fluid that does not induce significant friction losses during circulation and gives good hole cleaning at low pump rates even in high-angle wells. 

Selecting a reliable chemical formulation for the drilling fluid so that it exhibits the required properties is one part of the job. Maintaining these properties during drilling is another. 

Circulation of drilling fluid may be considered a chemical process with the wellbore acting as a reactor vessel. In this reactor, the composition of the drilling fluid will be changed dynamically by such factors as filtration at the wellbore and evaporation at the surface; solids will be added and taken away by the drilling process and the solids-control equipment; chemicals will be lost as they adhere to the borehole wall and to cuttings, aand they will be added routinely at the surface, formation fluids will contaminate the mud, perhaps causing flocculation or loss of viscosity and oxygen may become entrained.

Under these circumstances effective management is not trivial. Nevertheless, basic process control techniques have been applied rigside for some years to aid in the selection and maintenance of the fluid formulation and to optimize the solids-control equipment - such as shale shakers and centrifuges. This approach is often linked to incentive contracts, where savings in mud costs are shared between contractor and operator, and has led to remarkable savings in mud cost.

For example, with a systems approach to drilling fluid management for 16 wells offshore Dubai, mud costs were cut in half and reduced as a proportion of total drilling costs from 6% to 3%. At the same time, hole condition remained the same or better - this was assessed by looking at hole diameter, time to run casing and mud usage per foot of well drilled. 

Such an approach is based on three premises:
  • More frequent and more precise measurements, for example five mud checks per day and the introduction of advanced measurement techniques.
  • Efficient data management using mass balance techniques - which track the volumes of chemicals, hole and cuttings- and computerized data storage and acquisition.
  • Integration of the management of the solids control equipment with that of the drilling fluids.

Solid-control efficiency-the percentage of drilled solids removed versus the total amount drilled- is central to drilling efficiency and is a function of the surface equipment, drilling parameters and mud properties. For example, muds that have a lower tendency to hydrate or disperse drilled cuttings generally give higher solids-control efficiency.  

The significance of solids control is that penetration rate is closely linked to the volume of solids in the fluid. The greater the amount of solids, the slower the rate of drilling.  Mud solids are divided into two categories: high-gravity solids (HGS) comprising the weighting agent, usually barite; and low-gravity solids (LGS) made up from clays, polymers and bridging materials deliberately put in the mud, plus drilled solids from dispersed cuttings and ground rock.

The volume of HGS should be maximized, so that the total volume of solids in the mud is minimized, while still achieving the density required to control formation pressures. Therefore, drilled solids must be removed by the solids-control equipment. Howeer, some solids become dispersed as fine particles that cannot be removedd effectively. In this case, the fluid must be diluted with fresh mud containing no drilled solids.

But desirable properties are not always optimum ones. For instance, zero drilled solids at the bit is desirable. However, achieving zero drilled solids would increase mud costs dramatically. It is the job of mud management to plot the optimum course. To do this successfully requires accurate and regular input data.

Traditional field practice is to measure mud density and viscosity ( using a Marsh funnel) about every 30 minutes at both the return line and the suction pit. Other properties- such as rheology, mud solids, fluid loss, oil/water ratio (for OBM), pH, cation exchange capacity, and titrations for chloride and calcium- are measured once every 8 or 12 hours ( depending on drilling conditions) using 1-liter samples taken from the flowline or the active pit. These determinations are then used as a basis for mud treatment until the next set of measurement is made.

To gain better control over the mud system, a more meaningful monitoring strategy may be required. Simply increasing the frequency of traditional measuring techniques to at least five times a day and making sampling more representative of the whole mud system has improved control and significantly reduced the amount of chemicals used to drill a well.  

Mud solids monitor- A common indicator describing the solids content in the mud is the LGS-HGS volume ratio. This is traditionally measured using the retort,a technique that requires good operator skills, takes at least 45 minutes and often has an error margin of more than 15%.


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