O

requirements for balanced activity muds. (From Chenevert,37 SPE-AIME.)

Instead of equilibrating the chips in desiccators containing saturated solutions of various salts, it is sometimes convenient to equilibrate them against a series of solutions with increasing concentrations of sodium chloride, and against another series with increasing concentrations of calcium chloride. If the equilibrium water contents are then plotted against the salinities, the salinity of the mud required to balance the swelling pressure of the shale may be read directly from the curve at the intercept with the in situ water content (see Figure 8-40).

Salinity of Water in Desiccator, %.

Figure 8-40. Direct determination of mud salinity requirements for balanced activity muds (Schematic).

Salinity of Water in Desiccator, %.

Figure 8-40. Direct determination of mud salinity requirements for balanced activity muds (Schematic).

As already discussed, it is not possible to formulate a balanced salinity water base mud. Nevertheless the balancing salinity test should be made because the value obtained is useful for diagnostic purposes.

3. Swelling measurements. In order to measure physical swelling, the shale sample must be immersed in the test fluid so that cation exchange can take place. Linear swelling may be measured with a strain gauge.37 Volumetric swelling may be measured by confining a specimen in a cylinder with a piston, and observing the displacement of the piston as fluid is sucked through a permeable disc in the bottom of the cylinder.65 Alternatively, swelling may be measured by the linear displacement of a piston in the apparatus shown in Figure 8-40a.65a

Part A

Digimatic indicator

Part B Core Holder

Part A

Digimatic indicator

Part B Core Holder

Fluorescent Display for Measurement

Moveable Spindle Polyethylene Bag

Shale Test Sample Drilling Fluid

Stationary Threaded Anvil

Output to Miniprocessor

Fluorescent Display for Measurement

Moveable Spindle Polyethylene Bag

Shale Test Sample Drilling Fluid

Stationary Threaded Anvil r i

Figure 8-40a. Direct reading digimatic swelling indicator. (From Osisanya and Chenevert,65a Copyright 1987 by SPE-AIME.)

4. Dispersion tests. This test compares the degree to which cuttings will be dispersed into the mud by the drilling process. A weighed sample of dried cuttings, or core fragments, in a coarse size range is rolled for a standard time—the length of which depends on the dispersibility of the cuttings—at the temperature of interest. The mud is then screened through a fine screen, and the cuttings remaining on the screen dried and weighed. The percentage loss of weight is taken as a measure of dispersibility. The test is empirical, and any set of conditions may be chosen to suit local shales or problems. For instance, if the problem is an increase in viscosity caused by dispersion of the cuttings, 4 to 10 mesh cuttings might be chosen, rolled in the mud for eight hours and sieved through a 325 mesh screen. Another procedure, found necessary by Nesbitt et al66 when they observed that the active material in the shale was located in a network of fractures, was to roil large pieces of core and determine the amount remaining on a 5 mesh sieve.

The dispersion test is a valuable aid to mud selection; it is simple and quick, and has the great advantage that a number of candidate fluids may be run simultaneously.

5. Rigsite tests. Osisanya and Chenevert65a describe six tests that may be made at the rigsite to aid in drilling troublesome shales, namely swelling, dispersion, cation exchange capacity, hydration capacity, hydrometer, and capillary suction time. The first two are by far the most important. They state that all the tests are dependent on the particle size of the shale used.

Laboratory performance tests are helpful when choosing between several alternative muds, or in determining the optimum formulation of a particular mud. Such tests should be made under simulated subsurface conditions. Performance tests that consist merely of immersing shale specimens in candidate muds may give misleading results, because unconfined shale can disintegrate without developing any significant swelling pressure, whereas downhole the shale will not disintegrate unless swelling pressure develops sufficiently to increase the hoop stresses above the yield stress.

Performance tests may be carried out either in a microbit drilling machine or a model borehole such as that shown in Figure 8-23. In applying stresses, remember that the effective stress is the load minus the pore pressure, so that it is convenient to set the pore pressure at zero, and the vertical stress, the confining stress, and the mud pressure at their respective values at the depth of interest in the well, minus the pore pressure at that depth.

Specimens may be either cut from cores or reconstituted in a compaction cell (such as that shown in Figure 8-29) from slurries of powdered shale. Natural specimens represent underground conditions more truly, but—since no two natural specimens are exactly alike—each experiment must be repeated on a number of specimens, and the results averaged. Reconstituted specimens have far better reproducibility, but give only qualitative results, because conditions that have come about over millions of years underground cannot be reproduced in a matter of days in the laboratory.

The time required to compact a specimen increases sharply with increase in height of the specimen, because the basal surfaces compact first, greatly restricting the outflow of water from the center. A two-inch high homogenous specimen in approximate equilibrium with the compacting pressure can be made in one day. Performance tests may be used to compare candidate muds with respect to their cffect on; (1) the mode of failure, whether it be plastic yielding or caving in hard fragments; (2) hydration of the borehole walls, which may be determined by taking a sample of the hydrated zone around the hole and comparing its water content with that of the original specimen and (3) borehole diameter, which may be determined by measuring the volume of oil required to fill the hole. If the hole has enlarged so much that the specimen has collapsed, the time to collapse may be used as a parameter.

Planning mud programs is most difficult—but also most important—in newly discovered areas where much of the necessary information, such as lithology and pore and fracture gradients, is unavailable, and shale samples may be hard to obtain. Intensive sampling (preferably by coring) and laboratory testing should be done in early wells, and the results correlated with field experience. The information accumulated will save much time and money in later wells, and also reduce the amount of laboratory testing required.

It is hardly necessary to say that no mud will maintain a stable hole unless its properties are kept up to specifications. Frequent mud checks at the rig and remedial treatments based thereon are therefore essential. When drilling with polymer muds it is particularly important that the polymer concentration be maintained at the required level. Loss of polymer by adsorption on drill cuttings proceeds rapidly, especially when drilling rates are fast. As the polymer concentration drops, the rate of dispersion of cuttings increases, further increasing the rate of polymer adsorption. Polymer adsorption continues until the polymer concentration approaches zero, with consequent major destabilization of the borehole.

Good drilling practices are also essential to maintenance of hole stability. Experience has shown that much less hole enlargement occurs in troublesome shales if a straight hole is drilled and doglegs are avoided. Pipe speeds when tripping should be kept low in order to minimize transient pressures. High fluid velocities in the annulus will cause hole enlargement when drilling through rubble zones, and through highly stressed, spalling formations, and will exacerbate enlargement caused primarily by the interactions between shales and the drilling fluid, as discussed above. Erosion will be much more severe if the mud is in turbulent flow. Velocities in the annulus may be lowered either by decreasing pump speed or by installing smaller bit nozzles while maintaining the same pump pressure. It may be necessary to adjust the rheological properties of the mud to increase the cutting-carrying capacity, or to change the flow from turbulent to laminar (see Chapter 5).

Notation c = Cohesive strength K = Tensile strength pm = Hydrostatic pressure of the mud column

= Hydrostatic pressure of mud plus any hydraulic or transient pressures in the annulus pf = Pressure of fluid in formation pores

AP= Pw-Pi r — Radius at point of interest rw = Radius of gauge hole r0 — Radius of deformed hole

S = Overburden load, solids plus pore fluids

Pf = Density of formation water a - Effective intergranular stress

<To = Effective horizontal stress in virgin rock rr, = Greatest principal stress a2 = Intermediate principal stress a3 = Least principal stress ae = Hoop stress (or tangential or circumferential stress) <j, - Radial confining stress around borehole 0 = Angle of internal friction

References

1. Eaton, B.A., "Fracture Gradient Prediction and Its Application in Oilfield Operations," J. Petrol. Technol. (Oct., 1969). pp. 1353-1360; Trans AIME, vol. 246.

2. Hubbert, M.K., and Willis, D.G., "Mechanics of Hydraulic Fracturing," J. Petrol. Technol. (June, 1957). pp. 153-166; Trans AIME, vol. 210.

3. Freeze, G.I., and Frederick, W.S., "Abnormal Drilling Problems in the Andarko Basin," API Paper 851^41-L, Div. of Prodn., Mid-Continent meeting. (March, 1967).

4. Dickinson, G., "Geological Aspects of Abnormal Pressures in the Gulf Coast, Louisiana, 'AAPG Bull, vol. 37, No. 2 (Feb., 1953). pp. 410-432; Proc 3rd World Petrol. Cong., vol. 1. (1951) pp. 1-17.

5. Finch, W.C., "Abnormal Pressure in Antelope Field, N. Dakota," J. Petrol. Technol. (July, 1969). pp. 821-826.

6. Darley, H.C.H., "A Laboratory Investigation of Borehole Stability," J. Petrol. Technol. (July, 1969). 883-892; Trans AIME, vol. 246.

7. Stuart, C.A., Geopressures, Supplement to Proc. 2nd Symp. Abnormal Subsurface Pressure, Louisiana State Univ., Baton Rouge, La. (Jan. 30, 1970).

8. Koch, R.D., "Forties Field Involves High Angle, Gumbo"," Oil Gas J. (Dec. 13, 1976). pp. 70-78.

9. Burst, J.F., "Diagenesis of Gulf Coast Clayey Sediments and Its Possible Relation to Petroleum Migration," AAPG Bull., vol. 53, No. 1 (Jan., 1969). pp.

0 0

Post a comment