Case Study: Downhole Testing Tools for Formation Evaluation in High-Pressure and High-Temperature Environments

Abstract

The new era of hydrocarbon production and exploration has required continual technical change to enable the challenges presented by the more hostile environments into which exploration has now moved to be resolved. The new Brazil well environments include not only high-pressure, high-temperature (HP/HT) wells, but also extreme high-pressure, high- temperature (extreme HP/HT) wells. To test these wells, the projects have depended on improvement in available technology, since formation evaluation jobs now are required for the more difficult conditions in the high-pressure, high- temperature (HP/HT) wells. The definition of HP/HT well environments are those with bottomhole pressures above 10.000 psi and bottomhole temperatures above 300° F. When environments have bottomhole pressures above 15.000 psi and temperatures above 350° F, the wells are considered as extreme HP/HT. Until recently, downhole tools available for standard projects could function efficiently to approximately 350° F and up to 10,000 psi of pressure. However, special tools are now available for 450° F and 15,000 psi of differential pressure conditions. These tools could possibly function in higher pressures and temperatures, but often, the drill-stem test (DST) string design is limited by the capabilities of other components in the string. Challenges also concern memory gauges, which can work in temperatures of 400° F and 30,000 psi of absolute pressure, but real-time gauges available can only reach 350° F. For HP/HT wells, therefore, mechanical pressure and temperature gauges had to be used. There were also other challenges for downhole tools since the well-fluid type could cause some inconveniences such as solids decantation and low-pressure transmissibility that would affect the valves. Special attention also is required for the elastomers used, since they must be compatible with well fluids and temperatures. Now, however, thanks to the development of new technology, a formation evaluation job can be performed in extreme HP/HT wells, since tools and elastomers have now been developed that perform in a 20,000-psi and 400°-F environment; for 350°-F jobs, real-time downhole gauges are available; for 700°F wells, mechanical pressure and temperature gauges can provide P&T data that can be exported as an ASCII file, thanks to the new chart-reading capabilities now available. In Brazil's ultra deep wells, the projects now include formation evaluation jobs in HP/HT wells. This paper will discuss how formation evaluation jobs can be performed satisfactorily in extreme HP/HT wells with up to 20,000-psi and 400°-F environments by changing operational procedures for the tools and elastomers presently available. This paper discusses lessons learned that have provided: information for testing in extreme environmentswhich gauges are appropriate for specific conditionshow to perform a DST by adapting special procedures for use in hostile environment. The improved technology and information concerning testing in extreme environments have enabled improved drill stem testing operations to be performed in the extreme HP/HT reservoirs in Brazil using these methods.