Search

Coal permeability declines due to fracture closure during the dewatering stage. A new technique for stimulation of natural coal cleats through ultra-fine and ultra-light high-strength particle injection into a coal fracture system is proposed. Coupling this technique with hydraulic fracturing treatment resulted in particles entering cleats under leak-off conditions. The following optimal water-based coreflood experimental conditions were determined by applying Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to the interaction between glass particles and the coal matrix: stability of a particle-based suspension (no agglomeration); repulsion between particles and the coal matrix; and, immobilisation of coal natural fines. At these conditions, these particles were placed inside cleats and were not attached to the cleat entrance, leading to less external cake formation; no formation damage due to fines migration was observed. The experimental study was carried out on some bituminous coal samples. Micro-sized glass particles were injected into a coal core at minimum effective stress until core permeability decreased to a value predetermined by a mathematical model. An increase of the effective stress to its maximum value by injection of particle-free water resulted in an approximate three-times increase in coal permeability, when compared to the original value. The proposed technique can be used for stimulation of a natural fracture network in conventional and unconventional reservoirs, as well as for the enhancement of conductivity of micro-fractures around the hydraulically induced fractures. These particles can be used as a non-damaging leak-off additive during hydraulic fracturing stimulation treatments leading to long-term fracture conductivity.

Electrophoretic mobilities of carboxylate-modified latex microspheres and zeta potential of soda-lime glass beads were measured at zero ionic strength and high pH. Surface potential for latex microspheres was calculated using Ohshima’s theory for soft particles. Total potential of interaction between colloidal particles and engineered porous medium evaluated according to DLVO theory showed negligibly small secondary minimum, a high potential barrier, and a primary minimum. Such conditions favour particle-bead repulsion resulting in size exclusion being a major particle retaining mechanism. Normalised outlet concentration is inverse function of particle concentration. Concentration of captured particles reduced along the length of the column in downstream direction. An analytical model for deep bed filtration of suspension in porous media and straining under size exclusion capture mechanism is developed and validated by laboratory tests on suspension flow in engineered media. The model is successfully matched with the results from column experiments, predicting the suspended particle concentrations at the outlet.

Sulphate scaling may develop during offshore waterflood projects where mixing of injected and formation waters causes salt precipitation. Salt deposition results in permeability decline and, consequently, in impairment of well productivity. The problem has been widely presented in the literature for the North Sea, Campos Basin and the Gulf of Mexico. Recently several oil companies have reported several field cases of oilfield scaling in Australia. In this paper, methods are summarised for forecasting productivity decline from the history of well productivity index decline, produced water composition, and also from laboratory corefloods. The methods account for chemical reactions based on exact analytical solutions of inverse and forward modelling of quasi steady state oil-water flow towards producing wells. The main result obtained from the analytical model is a measure of the decline in productivity index with time for either linear flow in the case of a coreflood or the radial flow towards well. The analytical model has been used to predict further productivity decline in scaled-up producers of the deepwater offshore field X (Campos Basin, Brazil). Laboratory corefloods were carried out for field X cores and waters, and the model coefficients were determined. The productivity losses due to barium sulphate scaling have been noticed during several years of seawater flooding. The values obtained for reaction kinetics and formation damage coefficients are similar to those obtained from corefloods.