Exxon Company, U.S.A. has conducted over two-hundred drill stem tests from floating vessels in the Santa Barbara Channel, offshore California. The equipment and techniques which were developed and used in these operations are described. These provide a safe and effective means to evaluate formations by flowing test on a floating vessel and are applicable to other offshore areas. A new generation of tools which will further facilitate testing in rough weather areas is currently undergoing development.
Drill stem testing has been used for many years to provide information which supplements electric logging, mud logging, coring, and other formation evaluation techniques; in many cases, it pro vi des critica1 data which cannot be obtained by any other method. The primary objective of drill stem testing offshore, as on land, is the acquisition of sufficient data to determine formation productivity and fluid content through the measurement of flow rates, bottom hole pressures, and fluid properties.
The mechanical considerations involved in performing the test procedures are complicated by the motions of a floating vessel. To cope with this environment, several new tools have been developed: mudline safety valves, volume-balanced slip joints, slip joint safety valves, and annulus pressure operated tester valves. The fail-safe design of these tools and the redundancy of the overall test assemblies provides the necessary safety and reliability associated with the offshore environment. The system is complex and requires the supervision of trained technical and operations personnel. Their training should include study of the function and operation of each tool as well as the overall procedures.
Although it would be desirable, it is not possible to detail a down hole test assembly that would be optimum for all situations. Just as drilling procedures are tailored to meet the requirements of the well, a drill stem test program should be designed to satisfy the information objectives, the safety considerations, and the economic realities. In addition, there is usually more than one service company that is available and capable of meeting these needs. The tools utilized by these companies often differ in specific design and operation to accomplish the overall function.
The basis for the discussion that follows is one of the arrangements that has been successfully used by Exxon in the Santa Barbara Channel and should serve to demonstrate principles applicable to floating test operations in general. For reasons of improved reliability and favorable economics due to the higher frequency of misruns in open hole testing, all of Exxon's Channel testing was conducted in cased-holes.