Case History: First Isochronal Reservoir Test Successfully Performed Worldwide at a Bottomhole Temperature Greater than 430 degree F
Authors: Khati Milind V. , Pedrosa Luis , Farooqui M. Yusuf , Khan Zuber Ahmad 
Published: July 2010

Gujarat State Petroleum Corporation, Ltd (GSPC) recently decided to evaluate a reservoir in an offshore well on the Indian East Coast where the bottomhole temperature was higher than 433°F. The initial DST had shown significant potential for the zone of interest, and, therefore, GSPC decided to perform isochronal testing to obtain more detailed reservoir pressure information. In this environment, the DST tools would be required to function continuously in the high-temperature conditions for more than 16 days to retrieve the required data without interruption. These conditions would definitely challenge the capabilities of the currently available drill-stem testing (DST) equipment.

This paper describes how DST tools rated to 450°F-bottomhole temperature and 15,000-psi differential pressure were configured to maintain integrity and successfully evaluate the well. The tools were annulus pressure responsive (APR) and would be operated with pressure applied to the annulus from the surface. The string included a multi-cycle downhole tester valve, a multi-cycle circulating valve, a blowout preventer (BOP) safety valve, a tubing tester valve, a secondary circulating valve, and safety circulating valves. The multi-cycle circulating valve added the flexibility of displacing the string with nitrogen for well stimulation if the well did not flow.

The data describes the careful preparation of the tool string, the extreme conditions in which the DST tools had to operate to evaluate the well successfully, and how the meticulously-prepared tool string was capable of meeting GSPC's requirements. The DST tools performed satisfactorily for more than 16 days during the test.

This job set a world record for the service company for a reservoir test performed in these high-temperature conditions and may have set a world record for all testing operations performed by other service companies as well.

Advantages included elimination of coiled tubing, increased safety for this type of operation, and a reduction of rig time compared to other testing jobs.

Introduction

GSPC wished to evaluate a reservoir in an offshore well on the Indian East Coast where bottomhole temperature was in excess of 433°F, as earlier DST operations had shown that the area had significant hydrocarbon potential. As a result, GSPC decided to perform isochronal testing to obtain the detailed reservoir pressure information that would be needed to develop the area. The field location is shown in Figure 1. The basic well and reservoir information available before the testing just was performed is shown in Table 1. Figure 2 shows the reference data from wells in proximity to the subject well.

Traditional DST tools usually would not provide the results needed in an environment with extremely high temperatures for the length of time required to obtain all the data needed to evaluate the reservoir.

The standard set of DST tools with multi-cycle capabilities are usually not capable of realiably handling HPHT conditions. Standard DST tools are challenged severely by high pressure and temperature environments, as these extreme conditions affect the sealing barriers, metallurgy, and operating forces of tools, causing tool failures. These effects reduce the working pressure and temperature ratings of the tools.

To perform DST jobs in these extreme HPHT conditions, single-shot DST tools were used, as these tools involve fewer moving parts, do not contain a nitrogen chamber, and operate only once during the testing operations.

The major disadvantage with single-shot DST tools is they do not permit flexibility, redundancy, and thus, efficiency for the whole testing operation. DST jobs using these tools under extreme conditions can be performed, but it takes a longer time to achieve the test results.