Reservoir Compaction in Shallow Gas Reservoirs (2016)

This MSc thesis has been written by former intern Marloes Jongerius.

Compaction occurs in many consolidated hydrocarbon reservoirs and is responsible for both improvement and loss of recovery. This study assesses the effects of rock compaction and the consequences on the production and recovery in unconsolidated shallow gas reservoir a topic of which little is known.

The compaction behaviour in unconsolidated to weakly-consolidated reservoirs is studied by compiling a large data set from published laboratory experiments data. Analysis of the data set indicates that the initial porosity, the pre-consolidation stress and the degree of consolidation of the reservoir formation are the main controlling parameters on the rock compressibility.

To account for compaction in unconsolidated sands, a set of equations describing plastic deformation is derived fromthe equations describing linear elastic compressibility. The pore volume reduction and permeability reduction calculated based on these equations tend to over predict the effects of compaction when compared to measured data. Empirical relationships are derived for pore volume and permeability reduction based on compaction behaviour trends which can be applied to unconsolidated reservoirs.

With these derived empirical trends compaction is simulated on shallow gas reservoirs. A simplified reservoir model is built capturing the main characteristic of shallow gas reservoirs in the Netherlands. The integration of the effects of compaction within the simulations is done by incorporating the range observed in the pressure-dependent rock characteristics i.e. pore volume (porosity) and permeability. The assumption of a constant reservoir compressibility leads to an uncertainty in the recovery factor of up to 10%when compared to the pressure-dependent assumption.

Two effects of pore volume reduction are visible in P/Z plots (mass balance plots) of compaction-sensitive gas reservoirs: 1) the P/Z line decreases more gradually than when no pore volume reduction occurs, 2) P/Z line deflects downwards with ongoing production and deviates from the straight-line. The higher the pore volume reduction, the more pronounced is this bending of the P/Z-line. The effect of permeability reduction is not visible on the shape of the P/Z plot.

In all cases, it is observed that the effect of reservoir compaction is, with respect to the no-compaction case, positive for the recovery of gas. The predicted recoveries show differences which can amount up to 20% additional recovery. For a typical shallow gas reservoir of 1 BCM, these difference translate to a potential underestimation of 0.2 Bcmof gas produced.