Novel Approach to Data Analysis of Pressure Transient Testing in Fractured Carbonate Reservoir. Ansagan Field Case
Authors: Yeskozha B. , Aimagambetov M. , Kabyshev P. , Burlibayev A. , Kulagina T. , Kaipov Y. , Thomas D. P. , Zhandin A. 
Published: October 2017

The paper presents a new approach to the qualitative and quantitative description of a fractured reservoirs by integrating static fracture data and dynamic well testing data and results allowing the reservoir model to be updated to maximize the information value from well testing operations.

The work was carried out on the data of exploration well A-2, which was drilled in the large carbonate oil and gas condensate Ansagan field.

The approach to the analysis and integration of static and dynamic data includes the following –Analysis of core and formation microimager (FMI) data to build the distribution functions of fracture parameters, such as intensity, apperture and conductivity;–Constructing the different sensitivities of discrete fractures by using the stochastic modeling based on distribution functions of fracture parameters;–Forward modelling of pressure transient test response for each discrete fracture realizations by using new semi-analytical simulator;–Comparison of the simulated responses using a log-log plot from each realization of discrete fractures with measured data;–Estimation of reservoir and fracture dynamic properties by matching the simulated data with measured one.

In total, five zones were tested individually using a DST string in the Ansagan-2 exploration well. The well test program included flowing periods followed by Build-up (BU).

The well test response confirmed the complexity of reservoir structure and the dominance of flow regimes through a fractured network. The derivative-shape of all five zones showed different behavior depending on fractures distribution within the reservoir and their parameters.

Discrete fractures were built for each zone as a result of well test interpretation. The geological model with discrete fractures was updated within the radius of investigation of well test taking into account all available geological data. Each zone was characterized by both matrix effective permeabiltiy and fracture parameters, such as conductivity and length.

A 3D geological model was then built as a result of integration of all static and dynamic data which can dramatically increase the certainty in production forecasting and aid the decision-making process in the field development plan. Moreover, results of new methodology can then be applied to: –Well test design for future drilled wells to further increase the extracted reservoir information from an optimized build up length.–Planning of well stimulations, in this case acid jobs in a multi-layered reservoir to evenly distribute the acid effectively among the different reservoir layers;–Well completions to uniformly and optimally drain various reservoir layers.

This work uses a unique semi-analytical pressure simulator designed for fractured reservoirs, which allows the setting the fractures explicitly with defined dimensionless conductivity and length. The simulator helps to assess the influence of fractures on reservoir dynamic response and estimate the parameters of matrix and fractures, as well as the skin-factor.