Selecting Economic Refracturing Candidates

Abstract

Abstract Since the first fracture treatment was performed in 1947, hydraulic fracturing has been used to accelerate production and improve ultimate recovery in many reservoirs. However, many of the early fracture treatments resulted in wells which underperformed expectations. Naturally, refracturing has been proposed as a means of improving performance in these wells. Even with improved operational practices, fracturing techniques, and understanding, refracturing has been applied with mixed success. Our literature survey shows that refracture stimulation treatments in tight formations require increased fracture length and that refracture stimulation treatments conducted in wells in permeable reservoirs require increased fracture conductivity to be commercially successful. Further, refracturing in depleted reservoirs, though responding similarly, may not be commercially viable. We present simulation results and interpret field data which support these conclusions. Our contribution is to evaluate refracturing and provide guidelines to the working engineer with respect to refracture stimulation design and commercial viability. The paper is broken into three sections. These include a review of previous work, a simulation study of the refracturing process, and a section on the interpretation of field data where refraCturing has been applied. Introduction Success or failure of hydraulic fracture treatments is determined by post-fracture productivity. Prats showed that the steady-state productivity improvement of fracturing was related to the Dimensionless Fracture Capacity, FCD. This term can be described as the ratio of the fracture's ability to flow fluids to the wellbore, to the reservoir's ability to flow fluids to the fracture and is defined as: (1) where kfw is the fracture conductivity, k is the reservoir permeability, and xf is the fracture half-length. Figure 1 shows a log-log plot developed after Prats of the steady-state folds of increase versus Relative Conductivity for fracture half-lengths of 100, 500, and 1000 ft. This figure clearly shows that for low relative conductivities (less than 100 ft) post-fracture well performance is essentially independent of fracture half-length. Conversely, for high relative conductivities (i.e.. greater than 1,000 ft) post-fracture well performance is greatly dependant on fracture half-length. Note that Figure 1 is based on steady-state results and, therefore, neglects transient flow effects. As a result, this figure is best used to predict post-fracture performance in wells producing from permeable reservoirs where the transient flow period is short.