Manipulation of Coupled Osmotic Flows for Stabilisation of Shales Exposed to Water-Based Drilling Fluids

Abstract

Abstract Coupled osmotic flows have been studied as a means of stabilising shales exposed to water-based muds. The prime factor that governs the magnitude of chemical osmotic flow, i.e. the shale-fluid membrane efficiency, was investigated in detail. Its dependence on shale parameters, fluid parameters and external conditions was quantified. Membrane efficiency was found to increase with an increase in (hydrated) solute-to-pore-size ratio, with an increase in the shale's high-surface area clay content and with a decrease shale permeability when increasing effective confining stress. Moreover, new drilling fluid chemistries for improving the efficiencies of low- and non-selective shale-fluid systems were identified. Induced osmotic flow with optimised shale- fluid membrane efficiencies in water-based environments is presented as a new strategy for improving wellbore stability in shales. General Introduction Current environmental pressure demands replacement of oil-based muds (OBMs) which, for the last decades, have been the workhorse for the drilling industry and for shale drilling in particular. It is our view that the long term solution to shale drilling problems should come from cost-efficient, benign water-based muds (WBMs). However, conventional WBMs in general have failed to meet performance criteria in the past. which has led many operators to embrace synthetic mud technology. Synthetic systems indeed offer a convenient alternative presently. However, the direct costs associated with synthetics already weigh heavy on the driller's budget. Additionally. environmental legislation on organic loading is expected to become even stricter in future, leading to restrictions on disposal of synthetic fluids and cuttings, and the necessity for costly cleanup or alternative disposal. Pro-active improvement of WBMs is therefore highly desirable. Recent advances in the understanding of shale-fluid interactions have identified the mechanism of coupled osmotic transport in shales as a potential new lead for improving the shale-stabilising properties of WBMs. It has been found that shale systems can be rate-/ permselective to the transport of water and solutes (ions) and thereby may act as non-ideal membranes (shales are also known as "geo-membranes", see e.g. refs.) that sustain to some degree osmotic backflow of pore water induced by WBMs with water activities substantially lower than the shale. Such induced osmotic flows, which are a direct consequence of the chemical potential differences between the shale and the drilling fluid, can be manipulated to stabilise and strengthen shales. Background Hydraulic invasion by WBM filtrates in shales driven by mud overbalance is a major source of shale instability. Hydraulic flow may upset the delicate stress balance in a shale formation by enhancing near- wellbore pore-pressure and hydration stress, as well as affect shale strength by changing water content and cementation integrity (for a more detailed review on shale instability, see refs.). In order to improve WBMs, it is crucial that the flow of their filtrates into shales is reduced. Treating a shale in contact with a WBM as a discrete membrane system, the total flow into / out of a shale, JV, driven by differences in hydraulic pressure and chemical potential can be described by irreversible thermodynamics as: (1) where Lp is hydraulic conductivity (m2 Pa-1 s-l), x is the thickness of the membrane system (m), P is the overbalance pressure (Pa), is the membrane efficiency or reflectivity, and is the difference in osmotic pressure between drilling fluid and shale defined by: P. 497