Exploring and developing tight gas accumulations are both technically & commercially challenging due to the large subsurface uncertainty and low expected ultimate recovery (EUR) per well. In this paper, the appraisal phase of a tight gas discovery is described in three parts.
The first part highlights the main geological uncertainties associated with rock properties for tight sands with porosity as low as 5% and permeability of few tens to hundreds of microDarcy. As a consequence, free fluid contracts for such accumulations are difficult to pinpoint using conventional petrophysical analysis of open logs and due to the lack of reliable fluid pressure data. Tight gas accumulations are known to be associated with large gas-to-water transition zones, leading to much broader and uncertain estimates of the gas resources. This in turn adds to the complexity in identifying and planning development options for such discoveries.
The second part describes some new technologies adapted to narrow down the subsurface uncertainties and to speed up the appraisal phase of a tight gas accumulation. Such technologies include: under-balanced drilling (UBD), high resolution gas chromatography measured at drilling site, special core analysis and dedicated petrophysical evaluation for tight sands. In addition, new technologies related to seismic imaging and fracc optimisation are also considered with the ultimate aim of improving gas production rates and EUR per well.
The last part of the paper describes the underlying strategy in maturing a tight gas discovery into field development. The strategy in principle has three main elements: 1) narrowing down the subsurface uncertainty; 2) reducing well cost; and 3) improving expected ultimate recovery per well. The campaign takes advantage of the ongoing development of nearby gas fields that provides the opportunity for hooking up appraisal wells at an early stage for extended well test. This appraisal plan is managed and executed by a dedicated and well-integrated multi-disciplinary team.