Well Testing Analysis and Interpretation in Low Permeability Reservoirs: Theory and Applications for Tight Oil Reservoirs in Peruvian Northwest Fields

Abstract

This study shows the technical principles and methodology to design, execute, as well as do a reasonable analysis and interpretation of well tests in siliciclastic tight sands; Instead, Mini-fall-off pressure tests are the best alternative to assess earth-mechanical properties, to identify flow regimes (linear from radial), estimate reservoir pressure and effective permeability, and eventually, to calculate post-frac system permeability. In addition, rate transient analysis (RTA) incorporates dynamic surveillance data (pressure and production) to build a model allowing identifying boundary effects and predicting production performance with few history data. "G" function and After Closure Analysis (ACA) are used to estimate effective permeability and reservoir pressure, using surface pressure data from Mini-fall off test converted to bottomhole conditions. Then, pressure dynamic analysis is followed by using production data and wellhead pressure monitoring, with a "good-match" VLP correlation; Fetkovich's, Agarwal's and Blasingame's plots are prepared for a sound diagnostic to identify early time features (presence of micro-fractures, unusual wellbore storage, variable skin effect, flow regimes of hydraulic fracturing), verify reservoir model, and distinguish boundary effects such as: non-flow limit, presence of faults, barriers and lateral changes in reservoir properties. Finally, a type curve is prepared to forecast oil rates based on a prediction of wellhead pressure performance during lifetime. In general terms, mini-fall off tests allowed estimating reservoir properties of tight sandstones of Mogollón and Pariñas formation. With a reasonable degree of accuracy. Originally or partially depleted pressure conditions were able to be measured, as well as effective permeability estimations below 1 mD. In addition, half-lengths in between 70' and 100' were detected in most of the hydraulic fracturing jobs. On the other hand, in some cases, a micro-natural fractured system was identified during an early-time regime by a computer-aided-RTA-model with a reasonable match of Fetkovich's and Blasingame's plots. This behavior explains the high productivity indexes and initial rates founded in some hydraulic fracturing jobs, such as the case of well 3 on Peña Negra field; The RTA model shows an accurate history match (95%) after a two-year production phase. This methodology proposes the integration of mini-fall off tests to test and account for reservoir properties and rate transient analysis, to identify reservoir boundaries while monitoring production performance. The methodology incorporates and adapt the following techniques to low permeability reservoirs: "G" function and After Closure Analysis (ACA) to estimate effective permeability and reservoir pressureFetkovich's and Blasingame's plots to identify early-time featuresBlasingame's plot to model hydraulic fracturing features (half-length and width, Fc) and figure out boundary effects (faults, no-flow boundaries, etc.Type curves to predict production performance and estimate technical recoverable volumes.