Borehole stability

In the pre-development stage, core samples can be used to test the compatibility of injection fluids with the formation, to predict borehole stability under various drilling conditions and to establish the probability of formation failure and sand production. 

The caliper is used to correct the density and neutron porosity measurements for borehole effects and also can be used as a borehole stability indicator. Figure 4-289 shows an example of comparison between the MWD ultrasonic caliper and the four-arm wireline caliper run five days later. 

For many years uintaite and other asphaltic-type products have been used in water-based drilling fluids as additives assisting in borehole stabilization. These additives can minimize hole collapse in formations containing water-sensitive, sloughing shales. 

A borehole stability model has been developed that takes into account both the mechanical and the chemical aspects of interactions between drilling fluid and shale [1231]. Chemically induced stress alteration based on the thermodynamics of differences in water molar free energies of the drilling fluid and shale is combined with mechanically induced stress. Based on this model, it should be possible to obtain the optimal mud weight and salt concentration for drilling fluids. 

Further stability models based on surface area, equilibrium water-content-pressure relationships, and electric double-layer theory can successfully characterize borehole stability problems [1842]. The application of surface area, swelling pressure, and water requirements of solids can be integrated into swelling models and mud process control approaches to improve the design of water-based mud in active or older shales. 

Fiber-containing cement was initially developed as a high-strength material that could be used to line a borehole [1789]. Several relatively simple and cheap spin-off applications of fiber cement were identified, such as the use of fiber cement in cement plugs for borehole stabilization and as a lost circulation material. Several companies are already applying or offering fiber cement for these purposes in the field, in both organic fibers and metal fibers [372,1077, 1682-1684]. 

C. Durand, A. Onaisi, A. Audibert, T. Forsans, and C. Ruffet. Influence of clays on borehole stability A literature survey Pt 2 Mechanical description and modelling of clays and shales drilling practices versus laboratory simulations. Rev Inst Franc Petrol, 50(3) 353-369, May-June 1995. 

F. K. Mody and A. H. Hale. A borehole stability model to couple the mechanics and chemistry of drilling fluid shale interaction. In Proceedings Volume, pages 473-490. SPE/IADC Drilling Conf (Amsterdam, Netherlands, 2/23-2/25), 1993. 

Fam, M.A. Dusseault, M.B. 1998. Borehole stability in shales a physico-chemical perspective. Proc. SPE/ISRM Eurock 98, Balkema, Rotterdam, 461-470. 

Horsrud, P., Holt, R.M., Spnstebp, E.F., Svan0, G. Bostrpm, B. 1994. Time dependent borehole stability Laboratory studies and numerical simulation of different mechanisms in shale. SPE/ISRM 28060, EUROCK 94, Delft, ML, Balkema, Rotterdam. 8pp. 

A scale effect exists in borehole stability smaller diameter holes are more stable. Often, this is forgotten, and small diameter borehole strategies can be extremely successful in rapid deep drilling (advance rate is a function of borehole area). 

There are many mud systems and techniques available to increase borehole stability. In this discussion, all immiscible systems are considered to be oil-based muds others, air excepted, are water-based. Special chemicals such as liquid silicates that lead to precipitation of cementitious materials in the borehole wall are not dealt with here the mechanism appears straightforward, but success depends on diffusion as well. In most cases, the concentrations of the active agents must be controlled to remain above the levels at which beneficial effects ensue. High mud weights are not... 

This is essentially the observational approach in geomechanics. The economic rewards are large the cost of borehole instability worldwide is on the order of 3-4 billion, and if costs of blowouts that are in part facilitated because of borehole stability are included, the figure is far higher. 

Some general comments on borehole stability can now be made. Some of these comments can be generalized to the behavior of shales in other geomechanical conditions, such as in tunnel walls, dam foundations, and in weathering slopes. 

Thermal Stresses and Borehole Stability in Rock Mechanics. Proc. 2nd NARMS, Rock Mechanics Tools and Techniques, pp. 1121-1126. 

Down-hole pressure fluctuations, due to the termination of flow in the annulus, are unquestionably of great concern. When circulation is stopped, during a connection or survey, for example, the foam can break-back to soapy water and its gas phase. The gas can then expand toward the surface. In order to re-establish circulation of foam, the "broken-back water in the bottom of the well must be displaced by newly introduced foam through the drill pipe. The BHP increases until the fluid begins to move up the wellbore. As the fluid is lifted out of the well, the BHP decreases, subjecting the formation to a dramatic pressure drop. In pressure sensitive formations this may have an unfavorable effect on borehole stability. In these cases, special additives can be used to create exceptionally stable foams. These foams are many times referred to as "stiff, or "stable, foams. 

Mini Rigs are mounted on a trailer, truck, or a self-propelled ttack vehicle. These systems are designed for drilling in relatively soft semi-consolidated formations and are used primarily for installation of utility conduits and small diameter pipelines in congested urban areas. They are not suitable for drilling gravel, cobble, or other formations where borehole stability is difficult to maintain. 

Features Can be added to fresh water enhances the efficiency of cuttings removal increases borehole stability reduces the sticking tendencies of wet clays, thereby minimizing the risk of mud rings and wall packing Properties Lt. yei. transparent liq. sp. gr. 1.038 flash pt. 61 C pH (10% sol n.) 6.5-... 

Especially with the further development of the deviated and horizontal hole drilling technology, a large part of all off-shore wells are drilled with fluids based on mineral or synthetic oils, thus achieving the necessary lubrification and borehole stability. They may contain up to 50% emulsified water. The following products, derived from RR, are in use ... 

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