Abstract
For more than a decade, optimal value testing (OVT) has been advocated as a methodology that, in most cases, can replace conventional drillstem testing (DST) for in-situ permeability measurement. The term OVT refers to any pressure-transient test during which live hydrocarbons do not have to be produced directly to the surface (Elshahawi et al. 2008, 2012). Three primary types of well tests have been considered as part of the OVT toolbox: injection testing, wireline formation testing, and closed-chamber system testing with cleanup and repeat surges, which is a proprietary analysis technique.
This paper reviews the earliest OVT case studies retrospectively, with the benefit of several years of production. These cases include the first successful multicycle well testing/fluid-sampling closed system testing operation with near-emission-free and real-time data transmission to surface in deep water. Operations were conducted from a moored vessel without the use of a subsea test tree, setting a world record at the time for the maximum depth at which any vessel had been moored. The operation successfully gathered all crucial data; the overall test duration was shortened; and most importantly, safety and formation fluid handling were enhanced with no hydrocarbons offloaded or flared.
Data collected during all phases of the testing process were analyzed. These included the perforation, subsequent surge into the testing chamber, initial cleanup of flow/buildup, optimized closed-chamber surge flow, and final cleanup of flow/buildup. Standard techniques were used to analyze the traditional DST, but specialized techniques and software were developed to help plan and analyze the closed-chamber surge testing during perforation and surge testing. Data from the testing phase were used to generate an earth model to forecast production from the field. Several years of observed production confirm the early testing results and validate the OVT philosophy and the closed-chamber testing technology.
This paper discusses the testing protocols, optimized system design, and novel analytical techniques employed. It also compares pressure transient analysis (PTA) results obtained from surge testing, standard DST, and formation tester. The consistency of formation pressure and permeability measurements obtained from the various testing techniques and the agreement with actual production performance lends credibility to the results and confirms their viability for replacing conventional DSTs in many cases. Particularly in deep water, where cost and environmental constraints limit the feasibility of conventional DSTs and where early data gathering is essential, such techniques can provide a powerful complement—and often a viable replacement—to well tests.
