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
The formation damage caused by the injection of water containing suspended particles, which are stable and are not adsorbed spontaneously onto pore surfaces under Brownian motion, has recently been analyzed at a pore scale level. Formation damage is the result of four more or less overlapping successive steps:deposition on a grain surface,formation of mono- or multiparticle bridges with subsequent accumulation upstream from the bridges,internal cake formation as soon as the nonpercolation threshold has been reached near the core entrance, andexternal cake formation. The surface deposition is not uniform over the grain surface and varies from the upstream stagnation point to the near pore throat zone according to a function depending on flow rate and surface forces. The bridging of pore throats is strongly dependent on the effective pore throat-to-particle size ratio, and the pore-throat size is often reduced by previous surface deposition. A new model has been developed to predict formation damage while taking into account these different steps. The dominant mechanism in each step is governed by parameters that have a clear physical meaning. However, due to the complexity of natural systems, these parameters cannot be quantitatively predicted from theoretical considerations but can easily be determined by specifically designed lab experiments. The model predicts the retention by deposition, by bridging and by subsequent accumulation upstream from bridges, the concentration in flowing particles and the local permeability reduction as a function of the distance from the inlet, as well as the overall permeability reduction, and the beginning of external cake formation. This new model appears to be an effective tool for analyzing the consistency of a set of laboratory data and for selecting the values of the parameters that must be introduced in a near-well bore field simulator for the proper prediction of formation damage in a given application. Introduction The early models proposed in the oil literature aimed to fit the decrease in relative permeability observed when a particle suspension is injected into a permeable core. Thus these models are empirical or at best purely phenomenological in the sense that they simulate observed phenomena by using equations without any clear physical meaning. They are very attractive however for petroleum engineers since they are very simple and easily introduced in conventional field simulators without increasing computing time too much. A good review of these empirical models can be found in Ref. 1.
SPE Members Abstract To design acid fracturing treatments, acid fracturing models require acid leakoff values including experimental coefficients such as acid propagation rate and acid penetration distance into the formation. This paper presents an experimental study aiming at defining a methodology for the acquisition of these parameters. The pressure variation curve obtained during a conventional acid injection experiment in a Hassler cell is analysed in relation with the dissolution mechanism. X Ray Computerized Tomography is applied to characterize the dissolution patterns. Influence of core length, acid concentration and acid flow rate are considered/critical values are found above which the filtration process is dominated by the wormholing mechanism. Critical lengths and rates are interpreted in terms of the Damkohler Number, i.e. the ratio of the acid brought by transport to the acid consummed by the chemical reaction. In addition, from the interpretation of the experimental results, a new methodology is proposed for the measurement of acid propagation rates and distances. Introduction The rate of fluid leakoff to the formation during fracturing treatment is one of the most critical factors involved in determining fracture geometry for a given treatment design. In acid fracturing treatments, the acid reacts with the formation, inducing the creation of channels (wormholes) that in turn increase the fluid leakoff Attention has been recently paid to the modelling of acid leak-off. Settari proposed to multiply the leakoff velocity predicted by the conventional leakoff model by a factor determined in the lab as the ratio between the leakoff velocity for the acid and the leakoff velocity of an inert fluid. This factor is correlated with one or two quantities that are internally computed in the model -i.e. cumulative masses of acid spent and leaked off. Hill modified a conventional leakoff model by introducing a layer with no pressure drop. This layer corresponds to the region penetrated by wormholes. The leakoff is thus computed along several regions: the filter cake, the invaded zone that contains the wormhole region and the compressed reservoir region. In both cases Settari's model or Hill's model the wormhole propagation evaluation will play a major role and has to be fully understood and described. Depending on the injection rate, three different mechanisms have been proposed: compact dissolution, dominant wormhole and multiwormholes. At very low injection rate, the dissolution pattern is compact, most of the acid being spent at the entrance of the pores. As the injection rate increases, a dominant "wormhole" develops inside the core which propagation is controlled by the mass-transfer of acid on the pore walls. At higher flow rates, the pattern becomes more homogeneous resulting in severe branching and high acid consumption. P. 63
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