document

Paper Number	2007_KK
Title	Field Examples Of History Matching Of Formationtester Measurements Acquired In The Presence Of Oilbase Mud-Filtrate Invasion
Authors	Mayank Malik, Carlos Torres-Verdín, and Kamy Sepehrnoori, The University of Texas at Austin; Birol Dindoruk, Hani Elshahawi, and Mohamed Hashem, Shell International E&P
Source	48th Annual Logging Symposium, June 3 - 6, 2007 , Austin, Texas
Copyright	2007. Society of Petrophysicists & Well Log Analysts
Language	English
Preview	ABSTRACT The acquisition of fluid samples in hydrocarbon reservoirs drilled with oil-base mud (OBM) is challenging due to both presence of multiple phases as well as partial-to-complete miscibility between reservoir fluids and OBM. Throughout the samplingprocess, varying concentrations of OBM will lead to changes in observed (apparent) fluid properties. Similarly, sand-face transient pressure measurements are affected by OBM as the invasion process itself modifies both fluid viscosity and fluid density in the near-wellbore region due to mixing between different hydrocarbon components. We use a commercial adaptive-implicit compositional method to simulate the filtrate cleanup process during fluid sampling and compare the predicted pressure and apparent fluid properties at the sand-face against observed field measurements. A history matching approach is used to estimate formation permeability and permeability anisotropy. We apply the proposed workflow to three sets of transient field measurements of sink probe pressure, observation probe pressure, gas-oil ratio (GOR), and flow rate acquired with a formation tester in light-oil formations. Since the formation is invaded by oil-base mud filtrate, GOR can be used to diagnose types of fluids. We use a dimensionless fluid contamination function to relate transient GOR measurements to sample fluid quality. The successful comparison to field measurements validates our simulation model and helps us to diagnose and quantify adverse data-acquisition conditions such as plugging and noisy data. We perform sensitivity analyses to identify the dominant governing parameters such as formation properties, formation tester flow rates, relative permeability, and radial length of mud-filtrate invasion, on transient measurements of sand-face pressure and fluid contamination function. Our observation is that transient pressure, GOR, and density variations are sensitive to both the radial length of mud-filtrate invasion and the rate of fluid cleanup. If the radial length of invasion is large, then the total pumped volume must be increased in order to retrieve representative fluid samples. This can de achieved either by increasing the duration of the test, using higher rates of fluid withdrawal, or both. INTRODUCTION Wireline formation testers (WFT) are widely used to measure pressure, estimate reservoir permeability and permeability anisotropy, and to collect representative reservoir fluid samples. Often, WFT measurements remain influenced by the process of mud-filtrate invasion that takes place under overbalanced drilling conditions. When wellbore pressure is higher than formation pressure, the pressure differential causes mud filtrate to invade the formation. Invasion stops when a mudcake builds and prevents further flow of fluids. The recent increase in deep drilling operations around the world has popularized the use of OBMs (Cheung et al. 2001) due to faster penetration, inhibition of chemical alteration with shale sections, and good wellbore stability (Andrew et al. 2001). Thus, it becomes imperative to accurately model their effect on the invasion process and, subsequently, on WFT measurements such as sand-face pressure and downhole GOR. In the case of OBMs, the invading filtrate is partially to completely miscible with the native formation hydrocarbons. In addition, OBM induces changes in fluid viscosity, fluid density, and GOR. Due to the complexity of miscible flow in a probe-type WFT, limited work has been advanced to simulate formation testing in the presence of OBMs.
Number of Pages	16
File Size	1053 KB
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