Velocity Profiles In Perforated Completions

Abstract The productivity of perforated completions is greatly reduced when fines ace mobilized causing particles to plug pore throats within a formation. Even in clean sands, relatively large quantities of particulate matter are present Mobilization of these fines occurs as fluid velocities exceed critical levels which may be determined experimentally in tests performed on core samples. The results of these tests, however, provide linear velocities which must be correlated to account for irregular flow patterns in me near wellbore region. A three-dimensional simulator is developed to analyze the velocity profiles in the vicinity of a perforation. By assuming a simplified geometry in which multiple perforations are carried out at uniform vertical intervals in a radially symmetric manner, aximum velocities in the region surrounding a perforation are quantified. Thus, for a given injection or production rate, actual velocities can be calculated allowing direct correlation of laboratory displacement test results. Introduction The migration of small solid materials ("fines") within porous media has long been recognized is a source of potentially severe permeability impairment in formations containing oil and gas. Fines migration occurs when loosely attached particles are mobilized by fluid drag forces caused by the motion of fluid within the pore space. If a sufficient quantity of large enough fines are mobilized, they can collect at pore throats within the medium and cause permeability impairment. Muecke(1) documents how a variety of different types of fines such as clay minerals, quartz, amorphous silicia, feldspars, mica and carbonates can be susceptible to migration. Gray and Rex(2) reported the migration of mica needles and kaolinite. They also found that migration could be induced by alinity changes or abrupt reductions in the ratio of divalent to monovalent ions present in the brines. Morgan (3) investigated changes in pH and salinity with respect to fines migration. Muecke(1) documented how fines mobilization generally occurs only when the phase which wets the fines is in motion. For example, there are many cases of severe fines mobilization occurring in water-wet reservoirs during waterfloods only after the waterbreaks through to the producing wells. Other authors including Porte(4), Selby(5), Kwan(6), Prisholm(7), Gabriel(6), Borchardt(9), and Sengupta (10) have documented phenomena which affect fines migration. Gruesbeck and Collins(11) describe a detailed phenomenological theory of entrainment and deposition of fines in porous media. Laboratory Tests to Evaluate Fines Mobilization An important parameter to be determined in the laboratory is the critical interstitial velocity (actual velocity in the pore space) at which fines mobilization begins to occur. The tests are commonly conducted on short cylindrical core samples (generally 2.54 to 3.81 cm in diameter by 4 to 8 cm in length). The samples are generally clean and obtained from extracted core material. The use of extracted core material can, in ome instances, supply a "worst case" scenario. If a reservoir is oil-wet in the case of water injection, the migratory tines will generally be encapsulated and stabilized by the immobile or lessmobile oil phase and fines mobilization may not be as severe as indicated by the use of clean, water-wet core.