Unreliability with drill stem testing (DST) tools is often experienced by operators in the development of high pressure/high temperature (HP/HT) wells. These environments are normally defined as having hydrostatic pressures of at least 10,000 psi and bottomhole temperatures of at least 300 F. Factors affecting reliability may include deteriorating mud conditions caused by increased depth and temperature ranges, high pressures. which hinder tool operation, and reduced sealing capability resulting from the high temperatures. Solutions to these issues are available, but in many cases, are not used because of insufficient pre-job planning.
This paper discusses a joint project undertaken by eleven major operators and service/testing companies in the Norway, UK area to address these problems and to determine methods that would increase the reliability of DST operations in HP/HT well testing. Emphasis was placed on the selection of the downhole equipment used for the HP/HT DST's and design of the full scale testing of the North-Sea-area field tools in use. Additionally, an investigation was to be made to establish methods that could be used to verify and increase equipment reliability. The project also included design parameters for contingency conditions that were not normally present during equipment operations but could exist in emergency situations.
Eleven tools including tester valves, multiple-cycle circulating valves, bypass valves and safety/circulating valves from different service company/suppliers were rigorously tested. The results of the tests, which compare the susceptibility of various tool design types to reliability problems, will be provided as well as the types of tool modifications that were subsequently made to increase reliability of the equipment for the HP/HT conditions. The data presented will also include results of testing conducted on the equipment modifications.
The method of testing helped determine changes needed to address reliability in HP/HT conditions and has demonstrated that DST tools could maintain integrity in this type of environment.
For over 60 years, drill stem testing has been used to determine reservoir parameters of potential producing zones. However, the early DST applications were traditionally limited to open hole at low to moderate temperatures and pressures. These conditions would include temperatures between 150 and 300 F and pressures ranging from 1,000 to 10,000 psi. With the expansion of offshore DST to cased-hole applications with increased temperature ranges of up to 410 F and increased hydrostatic pressures of up to 15,000 psi, traditional equipment experienced operational problems. These problems included the following in descending order of relative frequency:
Mud deterioration resulting from mud remaining static at temperature for several days, either in water-based systems in which the settling of solids caused the string to become stuck in the hole and to create plugs of solids that hindered pressure transmission to the tools, or in oil-based systems that increased in viscosity and gel strength, reducing capability to transmit operating pressures to downhole tools.
Tool design that was not fully suited for the conditions. This included insufficient pressure capabilities, especially in the area of air chambers, shear pin-operated tools that were susceptible to pressure and shock from the perforating guns, and string designs that were too complicated for the conditions. For example, several annulus pressure-operated tools run with operating pressures that are too close together because of an insufficiently low pressure test of the casing or liner lap.
Both of the above problems can be minimized with the application of prejob planning.