Performance Evaluation and Formation Damage Potential of New Water-Based Drilling Formulas
- J.F. Argillier (Inst. Français du Petrole) | Annie Audibert (Inst. Français du Petrole) | Daniel Longeron (Inst. Français du Petrole)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1999
- Document Type
- Journal Paper
- 266 - 273
- 1999. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 5.2 Fluid Characterization, 2.7.1 Completion Fluids, 5.2 Reservoir Fluid Dynamics, 4.1.2 Separation and Treating, 2 Well Completion, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 2.2.3 Fluid Loss Control, 5.2.2 Fluid Modeling, Equations of State, 1.8 Formation Damage, 1.6 Drilling Operations, 2.5.2 Fracturing Materials (Fluids, Proppant), 1.6.9 Coring, Fishing
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Formation damage risks are well documented for standard mud formulations but they are poorly analyzed for new types of nonpolluting muds. This paper describes extensive laboratory work aimed at evaluating the behavior of these new types of mud formulations. First, static and dynamic filtration experiments were conducted on paper filters and rock slices. Examination of mud cakes by means of cryo-SEM has permitted correlation of filtration behavior with the structural characteristics of both external and internal mud cakes. Then, the simulation of the full process of mud invasion in oil-bearing reservoirs was achieved by performing static and dynamic filtration experiments in a specially designed core-holder cell containing 40 cm long sandstone core samples. Cumulative filtrate losses and pressure drops across six sections of the core, while circulating the mud and back flushing the oil, were continuously monitored to evaluate the permeability damage. Damage arising from overbalanced conditions has been evaluated in terms of cake permeability, fluid loss characterization and reduction in oil permeability after mud exposure. For the three mud compositions tested here, the filtration process in high permeability sandstones is mainly controlled by external mud cakes. Damage is severe but an additional damage due to the trapped aqueous filtrate phase can strongly affect the oil relative permeabilities. This approach has given a complete behavior understanding of new water based drilling fluid formulations including evaluation of their performances, limits of use at high temperature and assessment of risks for different operational conditions.
Water-based systems with enhanced properties are proposed to obtain more cost-effective production and to reduce specific well treatment operations. In particular, these fluids are formulated to provide lubricity, hole cleaning properties, and to control fluid loss. Water-base "drill-in" fluids are designed for open hole completion and specifically designed for ERD wells with long horizontal sections characterized by very long contact times between the drilling fluid and the formation that can adversely affect the productivity of the reservoir.1-3
An experimental approach was implemented to study formation damage due to drilling and completion fluids in downhole representative conditions.4 It includes detailed fluid characterization, static and dynamic filtration property studies, external and internal mud cake characterization by cryomicroscopy, filtration tests on long cores saturated with light oil, and determination of return permeabilities, and hysteresis of saturation during back flow. Objectives of this study are to better understand the mechanisms involved in formation damage caused by different water based muds on a model rock representative of a reservoir rock.
The formation damage mechanisms due to drilling and completion fluids are largely illustrated in the literature5-9 and can be summarized as follows:
Water-based muds (WBM) generally contain large concentration of bentonite that causes damage of the near wellbore due to the deposition of mud filter cake (both external and internal) followed by the invasion of the mud filtrate that can also damage the productive zone. The different formation damage mechanisms due to mud filtrate involve pore throats plugging by polymeric additives, ionic incompatibility between filtrate and formation water, and possible reactivity of the clays contained in the reservoir (swelling, dispersion,…). The tendency is now to use specific water based formulations named drill-in fluids10 containing sized mineral particles that can easily plug the pore throats thus providing instantaneous leak-off control.
Oil or pseudo-oil-based muds (OBM) present generally better filtration properties than WBM but may however be more damaging. The cake is generally constituted of fine water droplets stabilized by different surfactants within the oil phase associated with organophilic clays. As long as the emulsion is stable, the filtrate is composed of oil associated with surfactant. Formation damage in that case is generally due to emulsion formation within the reservoir and wettability alteration. An increase in oil wettability can decrease the oil mobility and affect oil production.5,6
Water Based Drilling Fluids.
Three water-based formulations, the compositions of which are given in Table 1, have been studied:
- MMH formulation is a mixed metal hydroxide mud formulated with bentonite, MMH, modified polysaccharide, and CaCO3 for bridging and weight control. 8,11
- HP/HT and HP/HT-d formulations are formulations proposed for high temperature conditions respectively non and weighted (780 g/L baryte). These formulations contain bentonite, a mineral viscosifier, a sulfonated terpolymer stable up to 170°C as a fluid loss reducer, a dispersant, and OCMA clay as drilled solids.
Formation damage due to an xanthan/bentonite formulation was previously described4 and the data will be used here as a reference. With this type of formulation the fluid loss control is performed by a relatively impermeable filter cake formed from clay particles and polymer chains network. 12,13
Static and Dynamic Filtration on Paper Filters or Rock Slices.
The API filtration cells were modified so that either filter paper or rock slice of 3 cm in diameter and 2-3 mm of thickness can be used. These slices come from the same rock material used in the long core tests and present a permeability to air of about 300 md. Before use, they are saturated with NaCl (20 g/L).
Dynamic filtration experiments were performed using a dynamic filter press with a cone/plane geometry. Rotation of the cone provides a shear rate that is uniform across the surface of filtration. This equipment has been described elsewhere.14 Filtration experiments were performed at room temperature with a standard pressure drop of 7 bars and shear rates at the surface ranging from 0 to 1000 seconds?1.
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