Consistency Check and Reconciliation of PVT Data from Samples Obtained with Formation Testers Using EOS Models
Authors: Kikani Jitendra , Ratulowski John 
Published: December 2007

Accurate description of hydrocarbon fluids is critical to field development planning including facilities design, recovery efficiencies and remedial procedures for intervention. Not only are a small number of downhole samples collected but often, they are not representative of the reservoir fluid for a variety of reasons. These include phase separation due to drawdown during sampling and leakage during recovery and transport of the sample. When a two phase sample leaks, the resulting loss of fluids (usually gas) causes the fluid to appear denser than the actual reservoir fluid. Routinely reported opening pressure is utilized for consistency check of the EOS characterized sample by performing an isochoric flash experiment which mimics the actual conditions. A significant discrepancy in modeled and real data often suggests leakage. Using the measured and predicted opening pressure, moles of lost gas are calculated and assuming the composition of lost gas the fluid is reconstructed. Using this technique multiple fluid samples from the same reservoir may also be reconciled and a consistent phase behavior of the fluids determined.

Introduction

Accurate description of hydrocarbon fluids is critical to field development planning including facilities design, recovery efficiencies and remedial procedures for intervention. Often, the samples collected downhole are not representative of the reservoir fluid for a variety of reasons. These include phase separation due to drawdown during sampling and leakage during recovery and transport of the sample. When a two phase sample leaks, the resulting loss of hydrocarbons (usually gas) causes the fluid to appear denser than the actual reservoir fluid. These fluids when characterized would underestimate the gas-oil ratio (GOR) or show higher condensate-gas ratio (CGR) than what the field may produce. This error can translate in undersized gas facilities and/or constrained oil rates from wells.

Often the only source of pressure and PVT data in a new field are samples obtained with a wireline formation testing tool. Surface recombined samples are available only for wells that are production tested. Wireline formation testing tools (referred to in a generic sense) are designed for multiple pressure measurements to establish fluid gradients and collect single or multiple fluid samples for fluid characterization. Some of the tools that belong to this class are RFT (Repeat Formation Tester), MDT (Modular Dynamic Tester), and RCI (Reservoir Characterization Instrument).

General sampling problems include seal failures and probe plugging that can occur in highly laminated or otherwise heterogeneous formations. Friable and unconsolidated fine sands add to the misery. In addition to sampling problems, fluid leakage and/or high formation drawdown resulting in two-phase flow may affect the use of a non-representative sample in laboratory measurements. Two phase flow into the sample chamber typically results in GOR's greater than or equal to the actual reservoir fluid GOR. If the fluid in the chamber is in a two phase non-equilibrium state when the sample chamber is closed downhole the opening pressure could be slightly depressed. As gas dissolves into the oil, pressure drops in the chamber. However, leakage after the fluid has cooled and broken into two phases results in lower GOR's and low tool opening pressure. We have observed this behavior for several cases recently in the Gulf of Mexico. In either case low opening pressure, is a warning flag for a non-representative sample. Often other data can help in determining the cause of a low value.

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