A Comparison of Methods for Calculating Pore Pressures and Fracture Gradients from Shale Density Measurements Using the Computer

Abstract

A Comparison of Methods for Calculating Pore Pressures and Fracture Gradients Pore Pressures and Fracture Gradients from Shale Density Measurements Using the Computer Four methods of calculating formation pore pressures and resultant fracture gradients have been adapted to the computer and are compared with an empirical correlation and electric log computations and with observed conditions such as mud weights used, gas kicks and other indications of pressure. Introduction The drilling of deep, abnormally pressured wells in the Gulf Coast area has brought about the need for accurate definition of the physical forces necessary to contain such anomalous pressure so that protective casing can be set and drilling can be continued without costly interruptions. It is not enough merely to recognize the presence of abnormal pressures. The magnitude of these pressures must be defined so that (1) drilling fluids can be selected to optimize penetration rate, (2) hydrocarbon shows from productive penetration rate, (2)hydrocarbon shows from productive formations can be recognized, (3) damaging fluid entry into potential pay sands can be lessened, and (4) electric log data can be interpreteted more reliably. Location of the transition zone between normal and abnormal pressures is of prime importance. Many excellent techniques have been developed to aid in its location and also to evaluate pressure requirements for drilling below these zones. Most of these tools, such as electric, sonic and density logs are used "after the fact" and are diagnostic only if run at the proper depth. Therefore, some method is necessary to provide information concerning impending high pressure provide information concerning impending high pressure zones as soon as they are penetrated by the bit. Two methods are in popular use today:normalized drilling rate or "D" exponent determination, which is premised upon a relationship between rate of penetration and differential pressure, andpenetration and differential pressure, anddetermination of the variation in the density of shale cuttings. Three modifications of the basic depth-of-sealing approach to pore pressure calculation using shale cutting density have been compared with two published empirical correlations. A GE Model 265 published empirical correlations. A GE Model 265 computer and GE's time-sharing service were used in this study. Basic Theory The most generally accepted theory explaining over-pressured reservoirs is based upon the Terzaghi and Peck perforated plate and spring model, Fig. 1A. From this analogy representing the shale compaction process, Hubbert and Rubey concluded that process, Hubbert and Rubey concluded that abnormal pressures occurred when water was not permitted to escape from a closed system (Stage A, permitted to escape from a closed system (Stage A, Fig. 1A) faster than overburden pressure was applied. Normally, compaction progresses to Stage C, or compaction equilibrium as defined by Hubbert and Rubey, where the fluids are under normal hydrostatic pressure and the overburden is supported entirely by pressure and the overburden is supported entirely by the springs or the grain-to-grain bearing stress of the rock. When normal leakoff cannot occur, as in Stage A, the overburden load is supported entirely by the entrapped water, and the fluid pressure reaches its ultimate value of 1.0 psi per foot of depth. JPT P. 1463