Production of oil and natural gas from challenging and unconventional (i.e. tight and shale) reservoirs has gained momentum and industry’s attention because of the important role they will play in fulfilling future energy needs globally. Predicting reserves and forecasting future production of these types of reservoirs is crucial for evaluation of new investments and auditing of previous expenditures. One rapid way of examining dynamic response of a reservoir using solely production data is decline curve analysis. When applied to tight and shale reservoirs, traditional decline curve analysis can lead to unreasonable results. The main reasons are the extended transient flow (caused by very low permeabilities) and reservoir heterogeneities such as layering and compartmentalization.
The above observations led to the research presented in this thesis. The study, consisting of two parts, focused on modeling and forecasting of long-term oil and gas production from tight and shale reservoirs.
In the first part, a capacitance-resistance model (CRM) was developed which was the result of stepwise coupling of the material balance and fluid-flow equations. Using capacitance and resistance terms, the concept of continuous succession of pseudo-steady states was introduced to enable using the depletion equations through a stepwise procedure for performance analysis during transient and boundary dominated flow (BDF). The model was then extended to multilayered and compartmentalized reservoir cases. Verification of the approach was obtained by comparing results against those of a reservoir numerical simulator. It was shown that the proposed compartmentalized model is able to consider the effects of low permeability barriers as well as different reservoir properties across the compartments. In addition, it was shown that performance behaviour of multilayered and compartmentalized reservoirs can be significantly different from that of a single compartment/layer reservoir; therefore the effect of additional compartments/layers should be accounted for.
In the second part of this work, an extensive simulation study was performed to elucidate the value of beta derivative for performing rate decline analysis. Based on the results, a new easy to use model for predicting future rate in conventional and unconventional oil and gas reservoirs was presented. Comparison of the model results with those of numerical simulation corroborated the reliability of the proposed method for forecasting production rates during transient and BDF.
This research demonstrated that physical complexities which are involved in production from the unconventional tight and shale reservoirs, including transient flow, layering, compartmentalization and natural fractures, can be simply captured by using the proposed approaches. They rely on simple reservoir engineering concepts. As a result, the proposed models offer new insights into production analysis of tight and shale reservoirs, using familiar and easy-to-understand reservoir engineering principles.