Mechanisms of Formation Damage by Retention of Particles Suspended in Injection Water. SPE Members Abstract This paper reports the main results of an experimental study of the process of permeability impairment by particles suspended in injection water. The experiments were carried out under well defined conditions both realistic and sufficiently well-known to yield clear and useful conclusions concerning actual retention mechanisms and their consequences for formation damage. Injection of waters containing particles of different sizes into sandstones of different permeabilities resulted in drastic permeability reductions, even for particles smaller than pore throats, with a very high flow rate sensitivity. The different possible steps in formation damage process have been characterized and related to phenomena at pore scale level. The results are analysed according to a new theory capable of predicting retention by two mechanisms for particles smaller than pore throats that do not absorb on pore walls without flow. This theory takes into account the competition between surface forces and hydrodynamic forces which can either induce or prevent retention depending on force balance at the location of capture. All experimental observations agree with this theory which, therefore, provides a reliable basis for quantitative predictions of formation damage. Introduction The retention of particles inside the reservoir rocks, which may cause substantial reduction in their permeability, is frequently at the origin of drastic declines in oil wells injectivity and productivity. In spite of its recognized importance, formation damage is still "a concept based on personal opinion and experience". The aim of this paper is to make clear what mechanisms are effectively responsible for retention, where retention takes place according to the type of retention involved and what are the consequences of this retention on permeability. Such a clarification gives the keys for a proper interpretation of experimental data and for a better prediction of formation damage characteristics as a function of the main parameters. The current approach of formation damage considers only two types of retention. The first one is surface deposition occurring spontaneously as soon as a particle arrives in close contact with a grain (or pore wall). This surface deposition requires surface attractive forces (Van der Waals electrostatic forces) or external forces like gravity for the layer particles. This type of retention has been studied extensively for particles much smaller than pores and reliable data and models are available, particularly for the kinetic of deposition onto isolated collectors and granular packs taking into account the respective effects of Brownian diffusion and convection. The effects of already deposited particles have also been taken into account. However, investigations concerning the case of particles having a size non negligible compared to grain size are quite recent. P. 329
A multidisciplinary study has been carried out so as to characterize the permeability impairment due to suspended particles during water injection. The approach is based on extensive laboratory experiments reproducing the complete range of parameters met in the industry. Experiments were carried out at both constant flowrate and constant pressure gradient. They reproduce static filtration conditions, i.e. without a flow component tangential to the rock face such as can occur when there are open fractures. The analysis of experimental results has confirmed that injectivity damage can be separated into two successive processes. Internal permeability damage close to the entry face switches to the build-up of an external filter cake after the injection of a critical volume. This is true even when particles are very small as compared to the size of the pore throats. Analytical equations have been developed for each mechanism, as well as for the critical cumulative injection when the external cake starts to form. These equations require at most two parameters. The extensive range of experimental conditions in our laboratory study enables us to propose correlations for the values of these parameters from basic data (velocity, concentration, particle size). Introduction Many oil fields, if not the majority, are produced using water injection which plays the two roles of sweeping oil towards the production wells, and maintaining pressure and therefore productivity at the production well. The quality of water to be injected has a major impact on the economics of waterflooding. Surface water treatment most often involves chlorination, filtering and deoxygenation. It is expensive both in capital costs, particularly topside when offshore, as well as increased operating expenditure. This leads to a strong economic incentive to reduce water quality to the minimum required. On the other hand inadequate water quality can result in dramatic formation damage, failure to meet the water injection target, and sooner or later loss of the oil revenues which were anticipated from waterflooding. Thus water quality is a compromise which needs to be optimized. Optimizing water quality requires a reliable prediction of formation damage as a function of cumulative water injection, for the different water qualities that can result from different treatment facilities, and in particular different filtration criteria. The purpose of this study is to propose simple analytical equations predicting formation damage due to the injection of solid particles, and this should help to optimize the design of water injection facilities. Review of previous models. Formation damage during water injection due to the presence of particles has been the subject of numerous publications, and the recent papers published by Van Velzen and Leerlooijer1, and Pang and Sharma2 present excellent reviews of previous approaches due to Barkman and Davidson3, Eylander4, Rege and Fogler5, as well as Todd et al.6. Van Velzen and Leeriooijer1 carried out a large number of laboratory experiments and interpreted the permeability damage observed in terms of the continuous growth of internal damage. A mathematical description of this process was proposed, making use of Darcy's law, the material balance for the solids in suspension and in the internal filter cake, and a modified Iwasaki7 relationship for deep-bed filtration.
A compact five-spot well pattern was completed through permafrost at Prudhoe Bay and circulated with hot fluid, creating thaw zones. Compaction Prudhoe Bay and circulated with hot fluid, creating thaw zones. Compaction and deformation of soil in these zones led to casing strain. Analysis determined that casing strains were caused by large differences in compactibility of soil types. Introduction The development of petroleum reserves in the arctic region of Alaska requires new petroleum engineering technology to deal with permafrost problems. A conventional well drilled through permafrost is subjected to the hazards of external freezeback pressures, internal freezeback pressures, and thaw consolidation (compaction). These three major concerns have been studied extensively by operators in the Alaskan arctic. Solutions to the problems associated with the freezeback of shut-in wells have been published in the past four years. Thawing of permafrost around a wellbore also can lead to casing damage. When ice melts, it is reduced approximately 9 percent in volume. Stresses carried through the ice phase of in-situ permafrost tend to transfer to the soil matrix as the ice melts and the pore pressure diminishes because of this reduction. The soil tends to compact when subjected to a higher stress level. The degree of compaction depends on the type of soil, the stress state in the frozen permafrost, the pore pressure after thaw, and the size of the thawed region. Although most compaction is accounted for by lateral movement of the surrounding frozen permafrost, some vertical consolidation or subsidence also can be expected. This vertical consolidation or movement can damage the casing strings throughout the permafrost section. The original field rules for Prudhoe Bay development required that a producing well be protected from subsidence damage by refrigeration, insulation, or other means. To understand thaw consolidation in the thick permafrost at Prudhoe Bay, BP Alaska Inc. conducted a permafrost at Prudhoe Bay, BP Alaska Inc. conducted a field test to thaw a limited region surrounding a well. The amount of thaw in this field test was equivalent to that created around a heavily insulated producing well. Both casing strain and formation movement were detected with a moderate degree of uncertainty in the field data. One solution to the internal freezeback problem is to replace water-base mud with a gelled, weighted oil-base fluid. The thermal-insulating properties of this fluid are quite good. However, a producing well insulated in this manner would develop a larger thaw radius during its operating life than was created in the BP Alaska field test. Petroleum engineers designing casing programs for field Petroleum engineers designing casing programs for field development were uncertain of the effect of this large thaw zone on casing integrity in a producing well. To assess the significance of the consolidation process for a gelled-oil-insulated completion, a major field test at Prudhoe Bay was conducted. The test site was located in Prudhoe Bay was conducted. The test site was located in Section 11, T10N, R15E, in the center of the Prudhoe Bay field and adjacent to a BP Alaska well where numerous cores were taken in the permafrost. Core analysis provided a complete lithology of the permafrost. The provided a complete lithology of the permafrost. The goal of the field test was to observe casing integrity and to measure the strain of a casing string surrounded by a thaw zone equivalent in size to a zone created by a well after 20 year's production. This paper describes the field test, operational aspects, and results. JPT P. 468
Studies of heat transfer and of the mechanical properties of permafrost and an analysis of stresses around refreezing permafrost and an analysis of stresses around refreezing wellbores have been combined to develop a computer program that calculates refreezing pressures. Calculated values agree well with values determined in a large-scale field test. Introduction Since the discovery of oil at Prudhoe Bay in 1968, much new technology has been developed for dealing with oil production in arctic regions. Part of this new technology has been the development of successful methods for drilling and completing wells through permafrost. Several previously published papers permafrost. Several previously published papers have dealt with the thermal aspects of drilling or producing warm oil through frozen soil, and in those producing warm oil through frozen soil, and in those papers it has been pointed out that some thawing papers it has been pointed out that some thawing around the wellbore is generally expected. If a well is allowed to refreeze, casing damage may result from two mechanisms. First, fluids confined within pipes and allowed to freeze may cause pressure to rise, damaging casing. Second, refreezing of thawed permafrost or fluids external to the casing may develop permafrost or fluids external to the casing may develop pressures high enough to cause casing damage. This pressures high enough to cause casing damage. This paper considers only pressures resulting from paper considers only pressures resulting from refreezing of permafrost and fluids external to the casing.As ice melts, its volume decreases by about 9 percent. If the volume decreases within the earth, there percent. If the volume decreases within the earth, there is a tendency for pressure within the liquid phase to be low. This tendency is offset by movement of fluid into the thawed region. We envision five possible sources of liquid that could flow into thawed regions:drilling fluid filtrate,water from below the permafrost,water from near the surface,brine permafrost,water from near the surface,brine moving laterally through permeable material into the thawed region, andliquid within the thawed region that becomes rearranged by gravity flow to resaturate deep thawed regions. If a thawed region is saturated with water and then allowed to refreeze, the system volume increases as water is converted into ice. Since permafrost refreezes most rapidly near the surface, excess water may be trapped when deeper thawed regions refreeze. Pressures rise, thus forcing liquid water to flow away Pressures rise, thus forcing liquid water to flow away through permeable material; if this is not possible, pressures rise until the excess volume can be pressures rise until the excess volume can be accommodated in some other manner. To insure the use of casing with the proper collapse strength, a number of studies of refreezing pressures have been undertaken.Large-scale field tests have been conducted in full-size wellbores penetrating the permafrost at Prudhoe Bay Prudhoe BayThe mechanical properties of permafrost have been studied.Pressures generated during refreezing have been measured in laboratory models.Theoretical methods for calculating stresses and pressures have been developed. In the remainder of this paper we shall report the results of those studies. We shall show a favorable comparison between experimentally measured pressures and calculated pressures. Finally, we shall give calculated pressures for some field cases of interest. Field Tests Two full-size wellbores were drilled and completed through the permafrost at Prudhoe Bay. JPT P. 1159
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