Effect of Porosity and Permeability on the Membrane Efficiency of Shales

Abstract

Abstract This paper presents experimental data showing the dependence of shale membrane efficiency of water-base muds on its petrophysical properties and mud composition. Wellbore instability often occurs as a result of osmotic pressures that develop when shale is in contact with water-base muds. The osmotic pressures generated are proportional to the shale membrane efficiency. It is not currently possible to directly measure the shale membrane efficiency downhole. A pressure transmission technique was used to measure the membrane efficiency of Atoka shale at different porosities. This provides a way to estimate shale membrane efficiency from wireline measurements. The results of this study could possibly be used to design muds that cause shale strengthening and result in better wellbore stability. Two series of tests were performed; the first used brine solution as the test fluid while the second used three industry provided water-base muds. Results show that the membrane efficiency of Atoka shale ranges from 0.4% to about 13% and is a function of the shale porosity. This shows that the porosity of the shale itself is an important consideration in mud design. The data clearly show that the membrane efficiency is negatively correlated with the shale porosity until about a porosity of 7.5% beyond which there is essentially no change in membrane efficiency. A good correlation was also found between the shale permeability (which is in the order of 0.1 nD) and the membrane efficiency. Beyond a permeability of 0.2 nD osmotic effects are small. For tests conducted with water-base muds, the membrane efficiency of the shale was reduced by a factor of more than 2 after contacting two of the three muds with the mud-altered Atoka shale. This decrease was found to correlate very well with an increase in porosity and permeability in the shale. However, muds which reduced the permeability recorded an increase in membrane efficiency. This shows the importance of porosity and permeability reducing agents in changing the membrane efficiency and osmotic pressure in shales. Shale membrane efficiency has been shown to correlate with the shale porosity and permeability. Interaction of the shale with different water-base muds is shown to change the membrane properties of the shale. Furthermore, the nature of this change determines the effectiveness of these muds in the stabilization of troublesome shales. This is important because of the time dependent nature of wellbore failure. This study shows that certain drilling fluids have the ability to alter the shale through permeability reduction induced by osmotic flow. The results of the study can be used to better design water-base drilling fluids that will stabilize shales. Background and Prior Work Shales are clastic sedimentary rocks containing lithified clay-sized mineral particles with distinct laminated layers. The clay constituents are often characterized by a large surface area and associated bound water. The clay types include smectite, which has water and sodium ions associated with it. It is also the most hydrophilic clay type and has the highest surface area (about 750 m2/gm). Illite is about four times less hydrophilic than smectite. It has potassium with no associated water. The surface area is about 80 m2/gm. However, kaolinite has no isomorphic substitutes in its structure and it is very pure with little or no reactiveness. Ceramists use it for this reason. The characteristics of any particular shale are often governed by the amount of each clay mineral present. Shales that contain more smectite tend to be very reactive and highly susceptible to swelling.