Gas production from fractured reservoirs can be very prolific, despite the extremely low matrix permeability that may be encountered. Knowing the limitations to production is important for operators who drill into very tight matrix reservoirs. To quantify production, the interaction between flowing hydrocarbons, water, and matrix must be well understood. In condensate reservoirs, where gas-liquid phase transition can significantly reduce production rates, the situation is even more complex.
The traditional approach to productivity assessments of fractured reservoirs includes using a detailed 3D reservoir model with detailed fracture network geometry obtained from separate mechanical modeling and/or field observations using microseismic devices and simplified analytical and semi-analytical equations. Although the detailed 3D analysis is both time- and resource-consuming and requires highly qualified personnel to run, the simplified analytical equations are not applicable for evaluating the condensate field production.
Objectives and Scope of Study
The primary goal of this work is to develop an application that will quickly predict pressure depletion and production decline of fracture-stimulated condensate reservoirs. For this purpose, an efficient numerical model was built, based on simplified parallel-planes geometry, but using a full three-phase transient analysis of pressure distribution in an extended stimulated domain to account for gas condensation and water flow. The effectiveness of the approach enables the use of this model as an application to existing wellbore simulators, reducing the need to combine them with a detailed reservoir simulator when a fast parametric or sensitivity analysis is needed.
In the effective approach to the problem, the computation domain is defined as half-space between two parallel fractures and extends from the wellbore to the boundary of reservoir. The model solves for gas, oil, and water flow parameters, and accounts for gas-oil phase transition. To solve the corresponding transient equations, the alternating direction iterations (ADI) have been used. The condensation/evaporation process is simulated using the PVT tables, which are downloaded before the simulations begin. It was assumed that the reservoir initially contained only dry gas and water, and the oil phase was produced during the pressure depletion in the vicinity of the fractures. Because of significant fracture lengths, the pressure distribution inside of each fracture is calculated by solving the lubrication theory equations. This solution is dynamically combined with a solution outside of the fractures; consequently, the pressure profile in the fractures is updated at every time step.
Results and Observations
The developed numerical model has been realized in MATLAB code and used for sensitivity analysis of reservoir productivity regarding changes of fracture size and spacing, as well as reservoir permeability. Because the model enabled phase transition between oil and gas, the banking effect (production reduction attributable to gas condensation) was identified and analyzed.
The obtained numerical model enables the prediction of the production decline in the fractured condensate reservoirs with a detailed account for multiphase reservoir flows and reservoir properties. The simplicity of the fracture geometry used makes the simulations fast and model usable in the form of application for a wellbore solver.
The developed mathematical and numerical model can be used to perform approximate production decline analysis with a detailed account for condensate properties, including the phase transitions. The simplicity of the model makes it possible to run this analysis in a form of an application for a wellbore simulator, reducing the need to combine it with reservoir solvers.
The pressure field and production rate can be calculated by using simple fracture geometry, but taking into account detailed physical-chemical properties of the condensate and material properties of the reservoir.