Drilling Fluids Approaches for Control of Wellbore Instability in Fractured Formations

Abstract

Abstract The presence of microfractures is now recognized as a common cause of severe wellbore instability, particularly in ultra-low permeability rocks such as shales and some limestones. In unstable microfractured formations, it is important to avoid pressurizing the fractures with drilling mud. This paper examines the rock mechanics aspects of wellbore instability in microfractured formations, and identifies drilling fluids and drilling practices that reduce the problem. Improved stability can be achieved by the use of certain drilling fluid additives that seal fractures. Limited success is seen with sized solids, asphalts, gilsonites or conventional lost circulation materials but a recently developed additive for use in oil- or water-based muds appears to prevent much of the instability normally observed. The mechanism of action of this additive is discussed and a recent field case history presented. Introduction Wellbore instability during drilling can be due to one or more of several reasons and may appear as cavings, tight hole or hole erosion. The consequences include poor hole cleaning (sometimes leading to packing off, stuck pipe and, ultimately, sidetracks), poor quality cementing, poor quality logs, inefficient perforating, difficulties running in and out of hole, and poor mud condition. Before action can be taken to improve stability, it is vital that the problem formations are located and the mechanisms of instability identified. Once the causes are understood, an informed decision can be made on how well planning, drilling practices and drilling fluid formulations can be modified to reduce problems and costs. Of all the rock formations that show severe instability, shale is widely considered to be the most common and most problematic. The frequent occurrence of instability in shales is due to a number of reasons:More shale footage is drilled than any other type of rockShales are mechanically weak and hence fail under lower stresses than, for instance, well cemented sandstones or carbonatesShales contain clay minerals that can react with some types of drilling fluidsMany shales are poorly cemented and hence easily eroded by flowing mudShales generally have very low permeabilities, often in the nanoDarcy range. This means that an effective mud cake does not form and so the pore pressure is not protected from the mud overbalance, as it would be in more permeable rocks such as sandstone1. The pore pressure will be increased in the near wellbore region2 and will not bleed off because of the very low matrix permeability; hence the effective stress reduces and the rock becomes weakerBecause of the low permeability and the often rapid rates of deposition and burial, shales can become overpressured with respect to other sediments in the geological column Therefore, before a shale instability problem can be addressed, a number of questions must be asked?Is it due to pore pressure (or overpressure) in competent shale? If so, raising the mud weight is a reasonable approach provided other formations exposed do not fracture, differential sticking risks in permeable formations do not become unacceptably high or - if the payzone is to be drilled - increased invasion rates do not cause formation damage.Is it due to chemical reactions between the drilling fluid and the shale? If so, can the water-based mud be modified or should a move be made to oil- or synthetic-based muds?Are the shales soft enough to be easily eroded by the circulating mud? If so, can the bit and fluid hydraulics be changed to reduce the problem?Are the shales fractured - either naturally or induced by the drilling process? If the fractures are induced, what can be done to prevent this happening? If the fractures are naturally occurring, what can be done to stabilize these fragile formations?