Field Example of Capillary Pressure Effects on Wireline Formation Tester Measurements and OWC Estimation in a Mixed-Wettability Oil Reservoir

Abstract

This paper discusses a field example of capillary pressure effects on down hole formation tester pressure measurements and oil-water contact estimation. Capillary pressure effects are investigated as functions of mud invasion, mobile formationwater, and rock wettability. Specifically, the effects of mudfiltrate invasion on formation tester probe pressures are quantified in terms of the error margin on the oil-water contact estimate. In the field example, a fluidpressure gradientof about .45psi/ft was established from the bottom to the top of the reservoir, which is indicative of a water bearing formation. On the other hand, bottom hole fluid samples with the Modular Formation Dynamics Tester (MDTTM) showed a 100%oil recovery in shallow intervals, moderate oil-cut recovery in the intermediate intervals, and lowoil-cut recovery in the deep intervals. The purpose of this paper is to use formation tester information from the field example well, offset delineation wells, and reservoir behavior information, to support and explain that the apparent gradient in this case is not a true representative of the fluid gradient in the reservoir, rather, it represents the water phase gradient created by filtrate invasion and mobile water. A simulation model was developed, based on reservoir properties and laboratory measured capillary pressures, to simulate the formation saturation as a function of mud filtrate invasion and mobile water. Log-derived water saturations were used as a guidance, and ultimately, the model was used to predict the apparent pressure readings by the formation tester. Together with the understanding of reservoir behavior derived from the simulation model, field data from all delineation wells were used to define the effects of mud filtrate invasion on formation tester pressure measurements. Consequently, the error margin on the oil-water contact estimate is benchmarked. Based on the field evidence, the paper also discusses which methodology is most accurate to use in the oil-water contact delineation in the field area of interest. Introduction In our field case example, oil-cut samples were retrieved from the formation with apparent water gradient in well A2, located in Haradh Increment-3 area, Arab-D reservoir. In order to shed some light on the observed phenomenon we shall present the results of the MDT job, identify the pressure points and zonations that samples were collected from, prior to the discussion and explanation of the observations. Firstly in the discussion, the definition of the oil-water contact, free water level, capillary pressure, and all relevant technical attributes will be presented. Secondly, the wettability of Haradh Arab-D rocks must be established. Then a field case will be used to illustrate the capillary effects of filtrate and mobile water upon the measured pressures using data from well A2 as well as data from relevant wells, e. g. A1, A3 and A4. Lastly, all reservoir parameters are put together in amodel to simulate the impact of capillarity on measured pressures.

Complexity of Wettability Analysis in Heterogeneous Carbonate Rocks, A Case Study

“…Al., 1984;Elshahawi et al, 1999;Elshahawi et. Al., 2000;Pham et al, 2005;Carnegie, 2006;Chenggang et al, 2006 andJakson et al, 2007) have commented on estimating accurate formation pressure gradient from the wireline formation pressure testers. One of the main conclusions from their works is that oil can be produced with a low water cut in a reservoir with measured water gradient.…”

Understanding the Pressure Gradients Improves Production From Oil/Water Transition Carbonate Zones

“…In some cases there is uniformly low resistivity (eg see [11] or [12]). This type of resistivity behavior is suspected to occur only in transition zones.…”