Relationship Among Earth Stresses, Pore Pressure, and Drilling Problems Offshore Gulf of Alaska

Abstract

Relationship Among Earth Stresses, Pore Pressure, and Drilling Problems Pore Pressure, and Drilling Problems Offshore Gulf of Alaska This paper presents an integrated analysis of borehole instability problems in the seismically active Gulf of Alaska. A well-documented instance of borehole collapse from one well is used to calculate horizontal stresses. For Gulf of Alaska, horizontal earth stresses were very, high and appear to have contributed to the occurrence of abnormal pore pressures and borehole instability. Introduction To date, drilling costs in the Gulf of Alaska have been high. Rig costs have exceeded $100,000/D in this hostile offshore environment. High fluid pressures have been encountered at relatively shallow pressures have been encountered at relatively shallow depths and have reached gradients as high as 0.89 psi/ft (20.1 kPa/m). Serious problems with borehole psi/ft (20.1 kPa/m). Serious problems with borehole instability have been encountered. These led to fishing operations and necessitated sidetracking operations in one of the two Shell-operated wells. Failure of the borehole wall by spalling, or sloughing, was indicated by caliper surveys and cuttings size. Penetration rates have been low, averaging less than 10 ft/hr (0.84 mm/s), over most of the footage drilled to date. All these factors, when combined with other operational problems, have resulted in a cast in excess of $1,000 /ft ($3,300/m) drilled.The wells drilled so far (Fig. 1) are near a region of active plate subduction and consequent high earthquake activity (Fig. 2). Normal faults are observed in this region, but only at shallow depths of 3,000 to 7,000 ft (900 to 2100 m). At greater depths, although there is some evidence of strike-slip faulting, thrust faults are the prevalent failure feature. This observed fault pattern suggests that at shallow depths the overburden, or vertical earth stress, is greater than the two horizontal stresses, while at greater depths the overburden is less than one or both of the other two earth stresses. 1 Consequently, the confining pressure (average of the three principal earth stresses) pressure (average of the three principal earth stresses) increases much more rapidly with depth here than in regions such as the Texas and Louisiana gulf coasts where only normal faults are observed and where lateral stresses are less than the overburden to great depth. Thus, sediment compaction may have proceeded quite differently in the Gulf of Alaska proceeded quite differently in the Gulf of Alaska than in the Gulf of Mexico. This may explain why methods that have proved useful in the U.S. gulf coast and other similar basins for predicting and detecting abnormal pressures generally have failed to respond in the expected manner to high pressures in the Gulf of Alaska. The high confining pressures also appear to cause the observed borehole failures; however, these failures allowed us to estimate the magnitude and direction of the previously unknown horizontal earth stresses. Regional Geology and Stress Field In the Gulf of Alaska, the Aleutian Trench is the ocean floor expression of the subduction of the North Pacific Plate beneath the North American Plate. Plate. JPT P. 1477