Damage Caused By Clay-based And Clay-free Inhibitive Fluids In Sandstone Formations

Abstract

Abstract This paper describes the results of a research project on the extent of damage caused by clay-based and clay-free inhibitive fluids in two consolidated sandstones of permeability ranges 0.008-0.013 µm2 (D) and 2.2-2.6 µm3 (D) under simulated borehole conditions using a closed loop circulating facility. The fluids, containing K +, Mg + +, or Ca + + ions as well as clays, solid bridging materials and different polymers, can be used either as drilling fluids or as workover, fracture and gravel pack fluids. The damage was studied on the basis of two factors, viz. damage ratio (DR) (percentage of original permeability lost after encountering the fluids) and sectional damage ratio (SDR) (determined by cutting the damaged core into several segments and then determining the damage ratio of each segment relative to the original permeability of the core). The sectional damage ratio is used to determine the radius and extent of damage in the formation. The dynamic filtration characteristics and the rheological properties of the used fluids were also evaluated. The effect of increasing the temperature and differential pressure on formation damage and filtration characteristics of the sed fluids were also investigated. The effect of the presence of formation pore bridging materials (viz. chalk) in decreasing the extent of damage is also demonstrated here. Introduction During the short phase of "mud spurt." a mud loss occurs in "overbalanced" drilling as a result of the positive differential pressure Δp. The mud spurt continues as long as free faces of the fommtion are available. After the building of the inner and/or outer "filter cake" only the filtrate can dissipate into the formation. In the mud spurt phase, the introduction of the solids into the formation, the changed conditions of fluid saturation in formation pores. solubilization of some of the materials from the rock matrix into [he filtrate and transportation of finer solids into the formation to a wide distance contribute to the reduction of formation permeability and hence well productivity; thus, the formation is deemed damaged. In addition to drilling, filtrate invasion and solid transport can occur during other well operations, such as well cleaning, fracturing and gravel Packing, which result in formation damage. This paper reports on the results of investigations into the extent of damage caused by some clay-based and clay-free inhibitive fluids containing cations and formation pore bridging materials in two consolidated sandstones. The effect of temperature and differential pressure as damage influencing factors were also investigated. The damage was evaluated using mostly 1–5% KCI-solution saturated cores and in some cases, oil saturated cores. Theoretical Background The reduction in the productivity of a well through contamination by drilling and treating fluids has been defined as formation damage. The productivity of a damaged formation is given by Muskat(6) as: Equation (Available In Full Paper) This equation shows that the productivity of a particular well in a formation depends upon two factor, viz. kd (permeability of the damaged formation) and rd (radius of the damaged zone).