Completion Design and Implementation in Challenging, Extreme HP/HT Wells in California

Abstract

As producing companies search for significant hydrocarbon resources, it has become necessary to pursue opportunities in frontier geologic horizons and geographic locations. This pursuit frequently results in encountering High Pressure – High Temperature (HP/HT) environments. The petroleum industry defines HP/HT wells as those exceeding 10,000 psi and 300°F. Several companies have drilled into HP/HT horizons in the California San Joaquin basin over the past 30 years, but generally operations were halted due to equipment limitations or due to limited hydrocarbon indications. In late 1998, a significant gas flow was identified from the Temblor formation at depths below 17,100 ft, with geologic information indicating a potential Temblor sand gross thickness of up to 3600 ft.. The pressure design basis for subsequent wells assumed an estimated equivalent pore pressure of 16.9 ppg. This information and other producing conditions, indicated potential bottomhole conditions of 425°F and 18,000 psi. Produced fluids also indicated the presence of hydrogen sulphide (H2S), which, at these pressures, dictate sour service metallurgical specifications. These potential extreme well conditions required very detailed completion engineering design, equipment qualification, rigorous planning, and precise field execution to achieve successful well completions. This paper will detail and discuss the well completion design basis and issues; equipment/perforating limitations and qualification tests, tubular stress and loading analyses, high-density completion brine usage, and actual field operational experiences. Numerous contingencies were planned in detail, some of which had to be implemented. The most significant contingency operation was a high pressure coiled tubing milling operation to clean out 2500 ft of formation and perforating debris, which plugged the tubing string. Background The wells discussed in this paper are located approximately 50 miles northwest of Bakersfield, California in the East Lost Hills Field. The East Lost Hills Field is situated in the southern end of the San Joaquin Basin, which began evolving during late Cretaceous and early Miocene time. The basin is primarily bounded to the North, East, and South by the granitic rocks of the Sierra Batholith and Foothills belt, and to the west by the San Andreas Fault. The primary objective of this drilling and appraisal program is the lower Miocene Temblor section (Zemorrian to Saucesian), which is a 3600 ft thick succession of interbedded sandstones and shales of bathyal origin. Wells thus far have reached depths between 17,400 ft and 21,700 ft. Pore pressure gradients at this depth are approximately 0.88 psi/ft, and reservoir temperatures are between 350–385°F. The first well was drilled in 1998 and penetrated the Temblor sand section; however, upon reaching the zone of interest, unforeseen pore pressures resulted in an uncontrolled flow from the well. The surface flowing information obtained from this well formed the basis for future well designs and equipment qualification requirements. Assumptions for the worst case reservoir pressures and temperatures of a well drilled to a total depth of 20,000 ft had to be made for the future well completion designs. The resultant maximums were extrapolated to be 18,000 psi and 425°F. These combined conditions, together with the fact that hydrogen sulphide (H2S) and carbon dioxide (CO2) were present in the well effluents, approach the operating boundaries and limitations of many tubulars and completion equipment.

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