Niche Reservoirs - Shallow Gas Fracturing; The Challenges and Learnings

Abstract

Production of shallow gas has presented a unique opportunity to implement a fit for purpose fracturing workflow due to the level of complexity these reservoirs present. Initially acquired logging data including open hole logs, mud logs, wireline pressure measurements and reservoir sampling as well as micro-frac readings confirmed the presence of relatively shallow gas in low permeability rock. Hence introducing fracturing as a favourable method of extraction made it imperative to address the level of complexity within the reservoir, which varied from the presence of anhydrites, extreme heterogeneity, water sensitivity, as well as the fault environment at such shallow depths. Exploring pilot holes and running advanced image logs as well as acoustic measurements along with micro-frac operation, provided critical data for completion design improvement to not only enhance the chances of successful placement, but also increase the overall gas output. The relatively low bottom hole static temperature and pressure, soft rock, heterogeneity and overall immaturity of the reservoir required extensive core flow tests. X-Ray Diffraction (XRD) as well as lithology scanner logs were also used to fully understand the complex mineralogy. A suitable salt tolerant fluid was proposed for fracturing before optimisation as well as the inclusion of fit for purpose acid systems. The workflow also utilised the extensive geomechanical datasets for analyses, as well as incorporating the geological and petrophysical interpretations. This was followed by sensitivity analyses of the fracturing design based on size of stages, stage spacing, cluster spacing, as well as the cement quality. After performing micro-fracturing tests, a one dimensional mechanical earth model (1D MEM) was optimised to enable better understanding the fracture geometry. The workflow also included the use of chemical tracers to qualify the success of each fracturing stage within the target horizontal section. The workflow started with a collaboration between geology, geomechanics, petrophysics, reservoir, as well as stimulation domains, which resulted in the completion of the first horizontal multistage fracturing completion within the targeted shallow gas reservoir. This milestone provided insight into the required planning for future gas wells within the region and has left significant potential for optimisation given the complexity of the reservoir. The consolidation of a workflow to deliver the first shallow gas project in order to extract the initially confirmed gas presence has presented a novel approach to such a niche project. This was initiated by utilising a time-lapse image analysis, petrophysical and reservoir evaluation, and then coupled with the introducing propped fracturing and matrix acidizing to further calibrate log-deduced parameters. A high level of detail in core analysis, as well as micro-fracturing interpretations, have reduced the uncertainty regarding fracture generation, initiation, and fracture extension into the far field in such a shallow and unconsolidated, low temperature and pressure reservoir.