Towards Zero Fluid Loss Oil Based Muds

Abstract

Abstract Fluid loss control is an important property of drilling muds which impacts the drilling operation in a number of ways including the prevention of differential sticking and formation damage avoidance. This study reviews the mechanisms of fluid loss control in oil muds. The main focus is an experimental study which probes the function of common additives such as barite and clays, dispersed liquid (the emulsion phase), and materials such as asphalts, gilsonites and polymers. HTHP fluid loss measurements and diagnostic techniques such as particle size measurement, SEM, and neutron scattering were performed to elucidate the mechanisms. It is shown that there are three critical mud components required to achieve extremely low fluid loss: fine clay, the emulsion droplets, and partially-soluble additives such as gilsonites or certain polymers. By optimising the mix, HTHP fluid loss values as low as 0.1ml/30 min at 120°C have been achieved. The gilsonites, asphalts and polymers do not necessarily function as deformable particles in oil muds - they partially dissolve and appear to act at the nanometre scale. Filter cake permeabilities are shown to be similar to that of a shale, hence the problem of obtaining zero fluid loss reduces to that of preventing fluid flow through shales but without the benefit of capillary pressure effects. Background Fluid loss control is a property of drilling muds that plays a varied and important role in the well construction process. Fluid loss and filter cake behaviour can influence aspects such as well bore stability, differential sticking, core recovery processes, downhole mud losses and formation damage. As a result, fluid loss control is important both for drilling performance and well productivity. A review of the SPE literature identified over one hundred and fifty papers relating to filter cakes and fluid loss, just in the last 10 years. The topics which have been covered include hydraulic fracturing and tip screenout studies, investigations of formation damage from filtrate or solids invasion, and various mathematical treatments of the filtration process. The mathematical studies consider aspects such as dynamic versus static filtration1,2, and comparisons of radial versus linear flow3. Our particular interest in this paper is to understand the factors which control fluid loss in oil muds. These systems produce low permeability cakes and a low fluid loss compared with water based muds, making them the preferred choice in many situations. Despite the large number of papers on fluid loss in general, the roles of the basic components of an oil mud - emulsion droplets, organophilic clay, lime, barite and other hard particles - do not appear to have been fully described to date. There are also questions which need to be answered about the mechanisms of additives such as gilsonites, asphalts and polymers. These materials reduce the fluid loss substantially in oil muds, but it is not obvious how they function when the fluids already contain such a wide-ranging mix of components. Certain studies in the literature explain the situation in part: e.g. Herzhaft4 investigated an organo-soluble fluid loss additive and gave a general description of the filtration process and the role of the emulsion droplets. Bloys5 studied oil mud filtration to minimise invasion during coring. It was concluded the key for low invasion was a system containing clay, 30–100ppb calcium carbonate, and a deformable fluid loss additive (gilsonite). The study did not consider the role of added barite or investigate the mechanisms in any depth. We wanted to explore the mechanisms in a systematic way and determine if current technology could be manipulated to produce ultra-low fluid loss systems. By improving understanding it could then be possible to extend the current technical limits, and produce fluids which have a positive impact on drilling and well productivity.