Abstract
In conventional reservoirs and certain tight reservoirs, both native reservoir quality and commodity pricing (via well economics) influence optimal well spacing outcomes. Optimized well spacing decisions in these reservoirs typically require case by case reservoir simulation studies and/or field trials both of which may be time consuming and impractical in the fast-paced unconventional reservoir development environment. In this paper, parametric reservoir simulation indicates that in reservoirs with formation diffusivity of less than 100,000 md-psi/cp commodity pricing and intrinsic reservoir properties have minimal effect on optimal well spacing. In these cases which include many shale reservoirs, the effective fracture half-length (Xf) is the singular determinant of optimum well spacing with NPV as the objective function. In these low diffusivity systems, well spacing optimization reduces to a geometric problem requiring predictive controls on effective fracture half-length. Classic rate/pressure transient techniques (RTA/PTA) are primarily diagnostic rather than predictive tools. Additionally, typical completions modelling tools are known to result in estimates of propped half-length which can be at variance with the effective fracture half-length (Barree et al. 2005, Rahim et al. 2012). This paper presents a predictive rate transient analytics (RTAN) framework within which the stimulated reservoir volume (SRV) quality and effective fracture half-length can be characterized as a function of 3D normalized treatment volumes (3DV) towards improved predictive controls on effective fracture half-length. Optimal prediction formalism is also introduced as a method to improve the predictive capacity of the 3DV parameter by accounting for geological and PVT variations. The combination of analytical well spacing modelling with rate transient analytics results in a data driven, integrated well spacing optimization workflow with reduced analysis time while preserving mechanistic integrity. Field applications in the Anadarko Woodford shale and Permian Wolfcamp are presented.
Introduction
The low permeability of tight reservoir systems such as shales ensures that in many cases the long-term drainage area is limited to the stimulated reservoir volume (SRV). The well density/spacing is therefore a critical parameter in optimal field development planning especially in terms of recovery and field NPV maximization. In general, optimized well spacing decision requires case by case reservoir simulation studies and/or field trials which account for varying reservoir properties and economic conditions. Lalehrokh and Bouma (2014) apply a reservoir simulation-based workflow in the Eagle Ford shale to provide well spacing guidance in the black oil and condensate windows. The study indicates that a well spacing of 330 ft and 400 ft maximizes the NPV of a black oil Eagle Ford. The study assumes an effective fracture half-length of 100 to 150 ft obtained from rate transient analysis but does not address predictive controls on the assumed effective fracture half-lengths. Xiong and Wu (2018) recognize albeit without explicit proof, the effective fracture half-length as the key driver of optimal well spacing. They therefore employ an integrated multi-stage fracture modelling approach to predict effective fracture half-lengths. The workflow implements discrete fracture networks (DFN), completions and production history matching and simulation. Although the workflow provides extensive mechanistic grounding, the time required to execute a single well study may prove impractical in the fast-paced unconventional reservoir development environment. Additionally, Xiong and Wu (2018) refer extensively to the hydraulic and propped half-lengths whereas it is the effective fracture half-length that is of interest in production and well spacing optimization problems. As illustrated in figure 1, Barree et al. (2005), Rahim et al. (2012) show that typical completions modelling based on completions history matching may result in estimates of propped half-length which can be at variance with the effective fracture half length.
