Investigation of Changes in the Structure of Clays During Hydrothermal Study of Drilling Fluids

Abstract

Abstract The need for a drilling fluid that will exhibit its basic requirements at temperature levels high enough to be used for geothermal drilling has prompted studies of various systems. The temperature range of some of these studies has been between ambient temperature and 700 degrees F (370 degrees C). A system was developed that exhibited the desirable properties of a good drilling fluid system at elevated temperatures and pressures. A fluid that exhibited these desirable properties after being subjected to high temperature and pressure was unusual as compared with presently used fluids. Therefore, investigation into why these fluids were capable of exhibiting good rheology and fluid-loss control was conducted. It was found that certain mineralogical components of the drilling fluid undergo transformation after being subjected to high temperatures and pressures. The conversion is temperature dependent but also is affected by the presence of various salts as well as the presence of silica and calcium.The fluids under investigation contained the mineral sepiolite as the basic ingredient. The changes in the sepiolite are reported in this paper. Introduction A drilling fluid (350 mL) containing sepiolite (15 g), Wyoming bentonite (5 g), NaOH (0.5 g), and organic polymers (2.5 g) was heated at 700 degrees F (370 degrees C) for 16 hours in a cement consistometer. In this system, after the same hydrothermal treatment, almost all the sepiolite was destroyed along with the carbonates and the 12.3- Na-montmorillonite was converted to 13.3- montmorillonite. A new crystalline phase was formed, constituting about 10 to 15% of the solids in the drilling fluid. This new phase occurs as laths elongation along the b axis, with lengths of 2 to 3 microns (2 to 3 m) and widths of 0.1 to 0.7 microns (0.1 to 0.7 m). The laths generally form regular aggregates similar to cordwood. In many of these stacks, individual laths are arranged in a regular manner into single crystals or into polysynthetic twins; some of them exhibit a perfect morphology with 60 degrees interfacial angles. The selected area electron diffraction patterns of the new crystalline phase displays an A-centered lattice with the unit-cell parameters: b = 7.17 0.05, c = 14.6 0.1 and = 90 degrees 10' 10'. These lattice parameters and the morphological features of the new crystalline phase are similar to those of xonotlite, a hydrated cement mineral.After these tests were completed, further studies were conducted using sepiolite alone in water and sepiolite with various salts added under controlled conditions. The usefulness of sepiolite in drilling fluids under extreme conditions of temperature, pressure, and pH during deep-well drilling now is recognized. Commonly, these drilling fluids are contaminated with salts of Na, Ca, and Mg from the formation. Sometimes the salts or hydroxides are added purposely to the drilling fluids. Therefore, it is important to evaluate the effects of these salts and hydroxides on the sepiolite under hydrothermal conditions. SPEJ P. 385＾