In-situ erosion of damaged formation during injectivity decline (PWRI & seawater flood)

Abstract

Injectivity decline with further stabilisation was widely observed and reported in the literature. It can be explained by different physical mechanisms. The correct diagnosis of formation damage mechanism allows choosing the right damage removal technology during seawater injection and Produced Water Re-Injection (PWRI). Reliable prediction of injectivity decline is essential for planning of well stimulation, fracturing, etc. Usually the stabilisation of well injectivity index after decline is explained by erosion of external filter cake, or by warm holes or even by fracture opening. Nevertheless, the stabilisation is observed in corefloods too, which evidences internal erosion. So, particle deposit erosion must be considered to interpret the injectivity stabilisation. In the current paper, the particle erosion was described by introduction of a new particle storage capacity function which equals to maximum retained concentration versus dimensionless flow velocity. After the maximum is reached by the retained concentration, particle capture does not happen any more. The particle storage capacity function is a reological characteristic that closes system of governing equations. The coreflood by suspension with permeability stabilisation was performed with a constant injection rate. The pressure drop on the core and the rate have been measured during the flooding. The analytical model developed allows to perfectly match the experimental impedance curve and calculate from it three injectivity damage parameters – filtration and formation damage coefficients, and also the maximum retention concentration. The obtained values of filtration and formation damage coefficients are in the usual variation range for these coefficients. Introduction of just one new parameter – maximum retained concentration – into a classical suspension filtration model allows for significant enrichment of the physics schema for suspension transport and retention. An analytical model of suspension coreflood with piecewise constant rate shows that after changing the flow velocity from some value and coming back to the same value, the impedance returns to the initial constant velocity curve. It takes some time after an abrupt flux decrease to stabilise the resistance growth rate, while the resistance growth rate stabilises immediately after abrupt flux increase. If flow regime changes from low velocity to high velocity, there appears a short particle pulse at the outlet; it does not happen when velocity changes from high value to low value.