Equation-Of-State Modeling For Reservoir Fluid Samples Contaminated By Oil-Based Drilling Mud Using Contaminated Fluid PVT Data
- Pashupati Sah (Calsep A/S) | Gurdev S. Gurdial (Core Labs. Malaysia Sdn. Bhd.) | Karen Schou Pedersen (Calsep A/S) | Hairul Izwan (Core Labs Malaysia Sdn Bhd) | Mohd Fadli Ramli (Core Labs. Malaysia Sdn. Bhd.)
- Document ID
- Society of Petroleum Engineers
- SPE Asia Pacific Oil and Gas Conference and Exhibition, 18-20 October, Brisbane, Queensland, Australia
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 4.1.9 Tanks and storage systems, 4.1.2 Separation and Treating, 1.11 Drilling Fluids and Materials, 5.2 Reservoir Fluid Dynamics, 4.1.5 Processing Equipment, 5.5 Reservoir Simulation, 5.2.2 Fluid Modeling, Equations of State, 5.2.1 Phase Behavior and PVT Measurements
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Bottom-hole samples collected in well-bore systems using oil-based muds (OBMs) are likely to be contaminated by medium to heavy hydrocarbon fractions present in the OBM. PVT data measured for a contaminated fluid will not be representative for the clean reservoir fluid and such PVT data is hence often ignored by the operator, which means loss of a considerable investment. It would be valuable for the oil industry to have options for numerical cleaning of OBM contaminated reservoir fluids and to be able to carry out Equation of State (EOS) modeling and regression for a contaminated composition in a way that would allow PVT data for a contaminated fluid to be corrected to represent the uncontaminated fluid. This paper describes such a methodology, which is integrated with EOS modeling procedures for numerically cleaned reservoir fluid compositions. Thanks to this methodology PVT data for contaminated samples does not have to be ignored and oil & gas operators can justify investing in PVT analyses for contaminated fluid samples.
The paper details the process through which the available data can be utilized. The composition of the reservoir fluid is estimated from the composition of the fluid with a certain content of OBM contaminate. A regression procedure is afterwards applied using the available PVT data in order to ultimately develop an EOS model for the clean reservoir fluid. Compositional data and PVT data are presented for a real reservoir fluid contaminated with OBM. Since also data is available for the clean reservoir fluid, it has been possible to verify the validity of the suggested procedure. The numerical cleaning procedure does not require any non-standard laboratory data and the given method is also not restricted to any particular brand of OBM or well-type.
Reservoir fluid samples are often contaminated by oil-based drilling mud (Gozalpour, 2002). In order to obtain a fluid composition representative for the reservoir the composition of the sampled fluid would need to be numerically cleaned. Historically the problem of contamination of bottom-hole reservoir fluid samples by Oil-Based Muds (OBMs) has been considered trivial due to the perceived simplicity of spreadsheet tools available for the numerical cleaning of the fluid compositions. However, the presence of the base oil contaminate may considerably alter the phase behavior of the reservoir fluid sample from that of the actual reservoir fluid. Additionally, in a majority of cases, the operator spends all available sample volumes as well as a part of the reservoir engineering budget to obtain PVT data for the reservoir fluid sample, which happens to be contaminated by OBM. The available data may be seen as useless and hence discarded when the extent of contamination becomes clear. This paper proposes that such PVT data for contaminated fluid samples can be corrected for the contamination to represent the actual reservoir fluid.
The OBM generally consists of components in the range of C11-C29, but are dominated by the paraffinic C11-C18 components. The numerical cleaning method requires the compositions of both the contaminated fluid and the contaminant (OBM). For a correct numerical cleaning it is important to obtain the composition of the OBM by having the PVT lab analyze a mud sample. For OBM contaminated samples the labs report level of OBM contamination as weight percentage of the stock tank oil (STO).
It is proposed that EOS models can be tuned to match the PVT data obtained for the contaminated fluid sample and the regressed component parameters applied for the uncontaminated/clean fluid.
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