Drill Stem Test Design Optimization Improves Quality of Reservoir Data and Time Requirements for Deep and Ultra Deep Water Well Testing
Authors: Cervantes Tomas , Bravo Gerardo , Loaiza Jan Rene , Ruiz Pablo 
Published: April 2012

Energy requirements are continuing to rise, and because the oil and gasindustry has been one of the largest contributors to the fulfillment of energyneeds worldwide, it is imperative that the technology needed to explore newarenas that can expand oil and gas production be available. Since explorationfor new reserves has had to move into more complex environments that includedeep and ultra-deep water reservoirs more difficult to access, all involved inexploration must also address the new challenges for deepwater. For operators, attempting to produce these areas represents huge investments for developmentof the new deeper reserves; for service companies, attempting to service thesenew developments means that new technologies must be available to ensure thatsufficient high-quality data is gathered in exploratory wells to justifysubsequent production. This requires that updates to technology beongoing.

Drill-Stem Testing (DST) provides one of the most important reservoirevaluation methods for the incorporation of new hydrocarbon reserves, becauseDST allows dynamic reservoir characterization and can assess potentialproduction as well as the proposed production plan. When target reservoirsproduce heavy oil, gas, and condensate, DST in deep and ultra-deep waterrequires prolonged periods of exposure to low temperatures and heat loss, whichmay not only affect viscosity for oil but also result in hydrate formation ingas presence; in order to control all these variables, the DST design mustfocus on gathering reservoir quality data that also consider safety and riskmitigation.

To achieve the above objectives, a new methodology that focuses on DST testdesign in deep and ultra-deep water has been developed that considers newworkflow criteria. It includes:Analysis of hydrate formationInjection design of viscosity-reducing systemsRealtime data acquisitionOperative decision making.

This paper presents details of this state-of-the-art method that enablesreliable DST testing to be conducted in deep and ultra-deep water and that canincrease the quality of the reservoir data as well as reduce testing time andoperational risk.

Introduction

Usually, exploratory wells drill many possible production reservoirs toevaluate the best exploitation scenarios, the number of production intervals, and their productivity potential. One of the most important areas in theevaluation is gained through well testing (Earlougher, R. J, 1977) (Soliman,M.Y., 2000). This is accomplished with a DST that consists of a pressure testusing a temporary string that evaluates all potential intervals that wereidentified through previously run openhole log results. Initial informationfrom seismic, perforation data, openhole logs and DSTs are necessary for athorough calculation of the potential reserves. The DST is particularlyimportant as it can obtain reservoir fluid samples at surface and bottomhole aswell, measure hydrocarbon rates, and reservoir border effects.

DSTs are initiated after the well is drilled to evaluate hydrocarbon reservesat deep and ultra deep sea beds. A temporary string composed of several toolsthat include a test packer, downhole shut-in valve, circulating valve andbottom pressure/temperature gauges is run. The DST will flow the reservoir, which disturbs reservoir pressure, and then, shuts-in the well to perform thebuild-up test.

Valves are controlled with annulus pressure; hydrocarbon and water rates aremeasured using surface well-testing equipment, since it is possible to takefluid samples at surface in real time in order to characterize the hydrocarbonand water flow. Figure 1 illustrates the equipment used in a typical DSTTest.