Pressure Transient Analysis in SAGD

Abstract Steam-Assisted Gravity Drainage (SAGD) is an in-situ thermal recovery technique used at Petro-Canada's MacKay River Project. SAGD utilizes pairs of horizontal wells placed near the bottom of the reservoir. Steam is injected into the upper well to reduce the viscosity of the bitumen. The bottom producing well collects the produced fluids. 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 well bore 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 falloff while continuing with production. Confined, unconfined and coalesced 2-dimensional sinksource 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 as well as determining recovery factors and possibly locations of reservoir boundaries as well as steam chamber coalescence. Introduction Steam Assisted Gravity Drainage (SAGD) is an in-situ thermal recovery technique that uses pairs of horizontal wells to produce bitumen. Steam is injected from the upper well into the formation to reduce the viscosity of bitumen. Gravity is the primary driving force as the steam rises and forms a steam chamber that gradually sweeps through the reservoir. The steam that is injected into the upper well creates a steam chamber that enables the heavier condensate and heated bitumen to flow downwards into the bottom producing well. (1) 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. 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.