Abstract

Evaluation of reservoir properties such as permeability, formation pressure and skin factor by means of welltest data interpretation is widespread. Usually these tests are implemented using pressure build up technique which leads to production losses. This fact makes regular well tests conductions to be undesirable by operators.

Another opportunity for reservoir parameters evaluation is given by transient period of well operation, the one starts after a new well start up and lasts until the pressure disturbance reaches the boundaries. In case when transient period lasts long enough (Priobskaya field) and production regimes are logged regularly, using this data seems to be reasonable.

A new approach to permeability, reservoir pressure and skin factor evaluation by production data analysis is presented in this study. This approach is based on broadening of the NODAL analysis concept onto transient case. The system operational point on the NODAL analysis plot is an intersection of inflow performance relationship curve and lift performance curve. During the transient flow regime, inflow performance curve changes with time. This allows developing expression for productivity index variation with time as dependent on permeability and skin factor. Matching the model curve of rate-time dependence to the actual curve allows resolving a system of equations containing reservoir parameters as unknowns.

Proposed method is approximate. A comparison to the rigorous solution for linear inner boundary condition is given. The results of calculation by proposed method were shown to be in good agreement to the ones obtained with the analytical solution. The case study including comparison to the build-up test also showed good performance of proposed method.

Implementation of the method proposed in this study allows, in particular, performing identification of transient regime, defining and ascribing production change to influencing factors and planning production optimization.

Introduction

Information on essential reservoir parameters - permeability, current reservoir pressure and skin factor - is vital for new wells introduction, optimization planning and field development control. Knowledge about current reservoir pressure may be obtained from the well test data interpretation or material balance equation; knowledge about permeability - from the wallets, correlations or laboratory experiments; knowledge about skin factor - from hydraulic fracturing design or from the well tests. Well tests are considered to be preferable method from all mentioned above cause they provide getting average reservoir parameters from the zone enclosing the well which are the necessary ones for production monitoring and control.

In order to maximize daily production rate under conditions of high oil prices operators tend to decrease the well tests number. Besides, because of several factors (not reaching the pseudo radial flow is among of them), quality of conducted well tests are not always acceptable. Two key factors - well tests number minimization and insufficient quality - decrease the level of knowledge about reservoir increasing uncertainty in optimization planning.

Using the most of information available from production data is seen to be valuable for reservoir parameters evaluation. Steady state regime is associated with constant rate, bottom hole pressure and productivity index. The well productivity in this case is described by simple Darcy's equation. For reservoir parameters evaluation however, data obtained from steady state flow cannot be sufficient. For getting information the well operation with different constant rates is necessary and each regime should last until stabilization. These conditions are not always easy to keep.

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