Simulation of Inflow While Underbalanced Drilling With Automatic Identification of Formation Parameters and Assessment of Uncertainty
- Torsten Friedel (Schlumberger DCS) | George Mtchedlishvili (ExxonMobil Corp.) | Hans-Dieter Voigt (Freiberg University of Mining and Technology) | Frieder K.A. Haefner (TU Bergakademie Freiberg)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 2008
- Document Type
- Journal Paper
- 292 - 297
- 2008. Society of Petroleum Engineers
- 5.1.1 Exploration, Development, Structural Geology, 5.1.5 Geologic Modeling, 5.5 Reservoir Simulation, 5.2 Reservoir Fluid Dynamics, 5.6.4 Drillstem/Well Testing, 4.6 Natural Gas, 1.4.3 Fines Migration, 1.8 Formation Damage, 1.6 Drilling Operations, 1.7.1 Underbalanced Drilling, 5.1 Reservoir Characterisation, 5.5.8 History Matching, 5.4.6 Thermal Methods, 1.11 Drilling Fluids and Materials, 4.1.2 Separation and Treating, 5.3.2 Multiphase Flow
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Underbalanced drilling (UBD) is defined as a drilling operation in which the pressure of the circulating drilling fluid is lower than the pore pressure of the target formation of interest. The most widely recognized benefit of UBD is the reduction of formation damage by minimizing the drilling-mud leakoff and fines migration into the formation. It also facilitates the possibility for reservoir characterization during drilling. The purpose of this paper is to present (1) how to use the inflow data for the evaluation of formation properties, and (2) how to cope with the uncertainty of the results.
An in-house multiphase reservoir simulator is used for the simulation of the formation reservoir-fluid flow during UBD. The model incorporates discrete consideration of the well with appropriate time-varying UBD boundary conditions. Capillary forces, which facilitate countercurrent imbibition of the drilling mud into the formation, are taken into account.
The production rates during UBD depend on the formation properties and the drilling conditions. The inflow information is analogous to transient test data and can be used for the estimation of the reservoir-model parameters. With the progression of the drilling process, the amount of available inflow data increases and allows for a sequential history-match procedure. Statistical analysis of the inverse problem can then be utilized to determine the optimal level of parameterization that is justified by the quality and quantity of the measured data.
This paper highlights (1) the use of a reservoir simulator to derive the inflow of fluids in heterogeneous reservoirs, and (2) its coupling with gradient-based optimization techniques for determination of unknown reservoir parameters. The simulation-while-drilling approach proposed is fully automated and can be set up on a known or totally unknown reservoir model, introducing an iterative and automatic updating process of the reservoir model.
In recent years, UBD has become a popular technology. Originally applied to prevent drilling problems, its value for reservoir engineering is now gaining greater importance. Benefits of UBD include the reduction of formation damage by minimizing drilling-mud leakoff and fines migration into the formation. Additionally, it allows for reservoir characterization during drilling.
Because the wellbore pressure is lower than that of the reservoir, there is a permanent inflow of reservoir fluids during UBD and, possibly, an outflow of drilling mud into the formation because of countercurrent imbibition (Friedel and Voigt 2004). The inflow rates can be obtained by balancing injected and produced fluids. This, of course, would assume that in-situ downhole conditions such as temperature and flowing wellbore pressure are continuously measured while drilling, which is not always the case.
By applying inverse modeling methods (assuming reasonably accurate data), it is now possible to determine reservoir parameters, such as permeability and pore pressure, by extracting the productivity signature of the reservoir from the production data. Although the short time span of the drilling process can give only an insight into the nearer wellbore vicinity, valuable information about the reservoir can be obtained. Productive reservoir zones and formation properties can be determined, provided there is proper flow monitoring at the surface. This offers significant benefits in both production optimization and reservoir characterization and can also be used to justify UBD.
The objective of this paper is to demonstrate the use of a reservoir-simulation tool, coupled with a method for parameter identification through automated reservoir characterization during drilling. In particular, the tasks from the reservoir engineering perspective, tracked here, are
- The detection of the layering,
- The identification of layer permeabilities (or other layer properties) and, optionally,
- Using statistical methods, it is not only possible to obtain important reservoir parameters but also to assign the corresponding uncertainties. In the modeling workflow, this data can be useful to validate the reservoir model and compare it to the initial geological model.
The workflow of the UBD-reservoir-characterization tool also enables "simulation while drilling.?? Its use is, therefore, not limited to the common post-drilling evaluation but, potentially, also engaged to optimize control during the drilling process. Optionally, the tool can be helpful for investigating reservoir formation damage as described in a previous paper (Friedel and Voigt 2004).
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