Pressure Transient Analysis in SAGD
Authors: Rabb J. , Palmgren C. 
Published: March 2010

Steam Assisted Gravity Drainage (SAGD) is an in situ thermal recovery technique used at Petro-Canada's MacKay River Project.

The objective of this study is to develop an analytical well test method that would allow a measure of steam chamber volume and potentially the timing and location of steam chamber coalescence. This well test method would also prove useful in determining an analytical solution for near wellbore characteristics and reservoir boundaries.

A pseudo-compositional thermal simulator was used to generate reservoir pressure responses by shutting off an injector at different periods of time and monitoring the pressure fall-off while continuing with production. Confined, unconfined, and coalesced 2-dimensional sink-source well pair models, based upon the geology and drilling pattern at Petro-Canada's MacKay River Project, were used in the study. The effect of the magnitude of the vertical permeability and the vertical to horizontal permeability ratio on the shut-in pressure response was also studied so that this method could be applied to other reservoirs of differing geology. Pressure response type curves were generated and the relationships between pressure drop at the injector, shut-in time, and steam chamber volume were determined.

We show that the pressure response type curves are unique for different types of SAGD wells and levels of steam chamber development. This information will aid in determining the timing of operational conversions and determining recovery factors and possibly locations of reservoir boundaries, as well as steam chamber coalescence.

Introduction

In a commercial SAGD project, it can be expected that as the steam chambers mature and sweep through the reservoir, the well pairs will begin to interact with each other and the steam chambers will coalesce(1). Coalescence is defined as the point at which separate adjacent steam chambers merge to form one large steam chamber. Coalescence is used for timing operational conversions to secondary recovery schemes, such as solvent injection. As a result of the planning that is required for the operational conversion, a method of measuring the volume of the steam chamber would prove useful in estimating the timing of coalescence and the subsequent operational conversions. Considering there are 25 well pairs at MacKay River, it is also necessary to know which well pairs have coalesced so as to know the well pairs in which to institute operational conversions. For this reason, a well test method for determining which well pairs have coalesced is also required.

The distance to and orientation of boundaries, flow regime, near wellbore characteristics (such as skin factor), and reservoir characteristics (such as effective permeability) can be determined through conventional pressure transient analysis. Conventional horizontal well testing has four different flow regimes that can be observed in the pressure response: early time radial flow, early time linear flow, pseudo steady state radial flow, and late time linear flow(2). The analytical solution is then applied in segments to the well test based upon the flow regime indicated by the pressure response. This segmental analysis can then be used to determine the reservoir characteristics.