The aggregation and deposition of colloidal asphaltene in reservoir rock is a significant problem in the oil industry. To obtain a fundamental understanding of this phenomenon, we have studied the deposition and aggregation of colloidal asphaltene in capillary flow by experiment and simulation. For the simulation, we have used the Stochastic Rotation Dynamics (SRD) method, in which the solvent hydrodynamics emerges from the collisions between the solvent particles, while the Brownian motion 2 emerges naturally from the interactions between the colloidal asphaltene particles and the solvent. The asphaltene colloids interact through a screened Coulomb potential. We vary the well depth ε and the flow rate v to obtain Pe flow >> 1 (hydrodynamic interactions dominate) and Re << 1 (Stokes flow). In the simulations, we impose a pressure drop over the capillary length, and measure the corresponding solvent flow rate. We observe that the transient solvent flow rate decreases when the asphaltene particles become more "sticky". For a well depth ε = 2 k B T, a monolayer deposits of on the capillary wall. With increasing well depth, the capillary becomes totally blocked. The clogging is transient for ε = 5 k B T, but appears to be permanent for ε = 10 -20 k B T. We compare our simulation results with flow experiments in glass capillaries, where we use extracted asphaltenes in toluene, reprecipitated with n-heptane. In the experiments, the dynamics of asphaltene precipitation and deposition were monitored in a slot capillary using optical microscopy under flow conditions similar to those used in the simulation. Maintaining a constant flow rate of 5 µL min -1 , we found that the pressure drop across the capillary first increased slowly, followed by a sharp increase, corresponding to a complete local blockage of the capillary.Doubling the flow rate to 10 µL min -1 , we observe that the initial deposition occurs faster, but the deposits are subsequently entrained by the flow. We calculate the change in the dimensionless permeability as a function of time for both experiment and simulation. By matching the experimental and simulation results, we obtain information about 1) the interaction potential well depth for the particular asphaltenes used in the experiments, and 2) the flow conditions associated with the asphaltene deposition process.
Summary Maintaining zonal isolation for the lifetime of oil and gas wells is critical. Leakage behind casing can reduce the cost effectiveness of the well and cause health and safety risks from pressure buildup and contaminated aquifers. During the completion and production phases of the well, temperature and pressure variations can cause stresses at the cement-to-formation interface. The ability of the casing-cement system to maintain a seal at the cement-to-formation interface depends on the condition of the formation surface before slurry placement. The condition of shale will depend upon the nature of the drilling fluid used, whereas the condition of a permeable rock will depend upon the presence and nature of the filter-cake deposited during drilling and circulation. In this paper, we present an improved understanding of chemical interactions at the cement-to-formation interface and the factors that determine bond strength and the position of the plane of failure. For permeable formations, the role of the mud filter-cake for different mud types is explored. For nonpermeable formations, the presence and effect of a mud treatment are also examined. The extent and depth to which chemical alteration of the mudcake occurs when in contact with cement is determined together with measurements of the yield stress and water-content profile of the altered mudcake. The effect on bond strength of exposing swelling and nonswelling shales to inhibitive drilling fluids is presented. Laboratory-scale test equipment and as mall-scale wellbore simulator, developed for tests under realistic field conditions, are described. A flexible (i.e., lower Young's modulus) cement plays a role in bonding and is demonstrated by the simulator tests. This improved understanding enables us to confirm the key issues at the cement-to-formation interface and propose some solutions for effective zonal isolation.
The interpretation of MWD and wireline logs of invaded formations can be enhanced by an understanding of the controls on invasion rates. Experimental investigation of dynamic filtration rates has shown that under certain conditions these are dependent only on the hydraulic shear stress at the mudcake surface for a given mud composition. They are virtually independent of differential pressure and independent of porous medium permeability (as opposed to mudcake properties) over porous medium permeability (as opposed to mudcake properties) over a 1:10 range. For very low permeability media however, the clear fluid Darcy flux may already be below the dynamic filtration rate. Then no mudcake forms and filtration rates obey Darcy's law. Comparison with theory suggests that there is a critical flux (filtration rate per unit area) above which day particles accrete irreversibly to the mudcake; below this flux they cannot stick at all. Invasion volumes at the time of wireline logging are usually dominated by fluid loss that occurred under conditions of dynamic filtration. The conjecture of a critical dynamic filtration rate, dependent only on mud composition and shear, means that invasion volumes should be independent of mud overbalance pressure and of formation properties (permeability, saturation, far field pressure, internal mudcake) unless formation permeability is so low that it limits the filtration flux to less permeability is so low that it limits the filtration flux to less than the critical flux. Then, formation properties control fluid loss. The permeability at which the behavior changes depends on both mud overbalance pressure and mud flow rate. These results are important for the interpretation of MWD and wireline logs in the presence of invasion. Field examples of multiple pass MWD and wireline resistivity logs and invasion volume computations from a single pass of wireline resistivity and porosity logs often indicate trends in the invasion volume within a zone or between zones. These trends may be interpreted as indicating differential formation resistance to invasion at low permeabilities but must be due to factors other than the loss rate at moderate to high permeabilities. One such factor is vertical movement of the filtrate under the Influence of gravity. Introduction Log analysts often seen to infer reservoir dynamic properties by interpreting the invasion profile at the time of wireline logging, or changes in the invasion profile between MWD and wireline logging times. An understanding of the influences on the rates at which filtrate is lost to the formation from the wellbore, particularly for those conditions under which the largest net volumes are lost, is a first step in this process. Fluid loss is conventionally divided into three categories: beneath-the-bit, static filtration and dynamic filtration. Fluid loss beneath the bit occurs as fresh rock is exposed. The loss rate is highest herb but ends within seconds, as pores near the wellbore are bridged, forming an internal mudcake. From published estimates one may conclude that the total invasion depths from volume lost in this phase are unlikely to much exceed half a wellbore radius. it is thus not of first importance for evaluating fluid loss at timescales exceeding half an hour. After the bit passage, further fluid loss involves mud filtration forming an external mudcake with concomitant invasion of the formation by mud filtrate. It is usual to distinguish two regimes: static and dynamic. These are really misnomers, since they refer only to mud flow in the well, not to the magnitude or time development of the loss rate. Dynamic filtration (referred to in chemical engineering as crossflow filtration) occurs when mud is flowing past the surface of the mudcake; static filtration takes over when the mud flow is stopped. For static filtration, the loss rate q(t) declines indefinitely as mudcake grows, in principle until the mudcake entirely plugs the wellbore. In dynamic filtration, q(t) declines until the mudcake stops growing and q(t) reaches (asymptotically) a steady state, with a nearly constant loss rate qD. Usually the majority of the fluid volume lost during time intervals of interest in formation evaluation is lost under dynamic conditions. This is because for most reservoir rocks, loss rates are greater for dynamic conditions than for static and because mud is circulating for the majority of the time for safety and hole deaning purposes. purposes. We describe in this paper simple semi-quantitative models for a complex process, concentrating on the principal features understood from existing literature and research, discounting effects and influences of secondary importance. Simple formulas are given; these deliberately sacrifice any attempt at high accuracy to serve as straightforward conceptual models for the major features of wellbore filtration. Most of what follows refers to orthodox water based muds, with bentonite as the primary viscosifier and fluid loss control material. More experimental data are required for other mud types to be confident in the applicability of these ideas to them. FLUID LOSS RATES IN DYNAMIC AND STATIC FILTRATION How the flow rate per unit area q(t) changes over time is significantly different between static and dynamic conditions for large times t. For small times however it turns out that the two regimes are closely similar. For this reason we consider static filtration first. P. 39
This paper was prepared for presentation at the 1999 SPE European Formation Damage Conference held in The Hague, The Netherlands, 31 May–1 June 1999.
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