Planning and Conducting Well Tests in Deep and Ultra-Deep Water to Mitigate Potential Risks and Justify Expense for the Operator
Authors: Wendler Curtis , Maia Claudio Tigre 
Published: May 2011

Well testing is a critical part of potential production assessment in deep and ultra-deep water well development. When target reservoirs produce heavy oil, gas, and condensate, or are high pressure/high temperature (HP/HT) environments, test designers must not only focus on safety and risk mitigation for the operator, when planning these tests, but also must assure that the tests will gather enough data to justify the operational expense and potential risks inherent to these types of environments.

Deep-water production usually requires prolonged periods of low temperature and heat loss, which can affect production or result in hydrate formation. Intervention can be problematic and costly. Fluid volumes, well and water depths accessed can increase well-control time and expense. Also, since these well tests are conducted from mobile vessels, the alarm and subsea equipment philosophies are critical to the enhancement of safety and risk mitigation. Well-test design and string configuration must have flexibility but still must be capable of efficiently controlling the conditions listed above.

Obviously, these and many other issues must be fully understood by the operator if the program plan, equipment, and methodology have the capability to assess the challenges, ameliorate the risks, and provide contingencies for unexpected events that could occur.

This paper explores these issues as well as appropriate methodologies for mitigation. The merits and limitations of various solutions will be considered along with the basic philosophy of equipment consideration. Lessons learned from actual cases will be used to compare consequences of inadequate preparation to the benefits of proper design.

Also explained will be why particular methods and equipment should be used and why others are less desirable. Lessons learned and their application will be discussed, including:Red, yellow, or degraded status decision making during dynamic-positioning (DP) vessel testingCoiled-tubing usage criteriaDefining cushion-type criteriaDefining mud-type criteriaHydrate prevention and mitigationImportance of obtaining dynamic reservoir data efficiently and economically without risking the environment and personnel safety.

Introduction

For purposes of this paper, deep water is defined as a water depth in excess of 1,000 feet or approximately 300 meters while ultra-deep would refer to water depths in excess of 5,000 feet or 1,500 meters. (Curtis, et al., 2003) Well testing is considered as a temporary completion of a producing reservoir with flow through a surface train for the purpose of measurement to some kind of disposal system. This process most often involves flaring, but in some circumstances, may simply be tankage of some kind - either using portable, rig mounted systems or in a separate vessel. Dynamic data and representative samples would be the expected result of such a well test.

Though water depth and associated " metocean?? conditions are the primary challenges associated with operations of this kind, additional consideration must also be given to the common complications imposed by attempting to produce unconsolidated reservoirs that might require sand control; heavy oil reservoirs with an API gravity of less than 20 degrees, a high specific density, or most critically, a high viscosity; gas and condensate reservoirs or reservoirs with bottomhole pressures in excess of 10,000 psi; and/or temperatures in excess of 350º F.