After Closure Analysis an Underutilized and Undervalued Approach to Understanding kh

Abstract

Abstract For low permeability tight gas reservoirs, the fracture half length and the fracture dimensionless conductivity are the two principal design parameters, goals and characteristics that can be set; that have the greatest impact on the actual well productivity index. The fracture dimensionless conductivity compares the ability of the fracture to convey fluids along its length into the wellbore, versus the capability of the reservoir to support the flow of fluids into the fracture itself. However, in most cases, and certainly in an exploration and appraisal environment, the formation permeability and effective permeability thickness (kh) are often misunderstood, miscalculated and misapplied; subsequently impacting the fracture design process and therefore the overall well performance. A large part of this exploration and appraisal process includes understanding the nature of, and gathering accurate data on, the reservoir performance itself; so that engineers can make the necessary informed decisions. In terms of estimating an accurate value for the effective permeability, techniques such as pre-frac pressure build ups and pressure draw-downs are probably the most common approaches taken; however these approaches can be operationally time intensive and costly, awkward to effectively execute and the data open to wide and non-unique interpretation. As a result, the industry developed (some time ago), the little used process of pre-frac injection tests; to help identify formation transmissibility and reservoir pressure. This operation consists of the injection of a small amount of fluid into the formation at low rates and accurately recording the formation response during the pressure decline. From analysis of this data, it is relatively straightforward to calculate the effective formation kh (to reservoir fluid), optimize the hydraulic fracturing design, improve the reservoir model and more accurately perform post fracture well diagnostics. This paper presents the application and results of such after-closure analysis (ACA), the impact of these calculations on the subsequent fracturing operations and finally their integration into a more comprehensive understanding of the fracture/reservoir performance and deliveribility.