Traditionally the oil and gas industry has identified formation damage as the one caused by the drilling process such as filtrate and solid's invasion. Other types generated during the completion and production phases normally occur while perforating or during hydrocarbons production, typically fines production, asphaltenes deposition, changes in formation wettability and scaling. On the other hand, mechanical damage defined as that induced by stresses is seldom considered or even measured, it is poorly understood and very difficult to identify. Solid's invasion from the drilling fluid and permeability change at the sand face are the two damage mechanisms discussed in this paper. Both mechanisms are treated using conventional filtration theory and plastic rock behaviour principles.
On this basis our paper presents a methodology developed for the identification, quantification and removal of formation damage caused by these two mechanisms in new wells. Quantification of both types of damage on a continuous basis throughout the sandstone is carried out to determine the location and severity of the combined mechanisms.
Introduction
The drilling process involves the cutting and removal of rock that is transported to surface by the circulating fluid. In most circumstances the pressure exerted by the fluid is higher than that of the formation establishing a pressure over balance condition between the circulating pressure and the reservoir pressure. In porous and permeable sandstones, this over balance condition induces the penetration of drilling fluid into the rock matrix. Fluids and solids penetrate the matrix invading it until the solids contained in the mud plug the pore throats forming a semi permeable barrier known as the mud cake. The depth of invasion for both fluid and solids is dictated by the rock properties, pressure differential and the mud properties. Filtration theory and plugging 1,2 principles are used to characterize the invasion mechanisms, type and penetration of the fluids and solids into the matrix.
At the same time the pressure exerted by the drilling fluid tries to compensate the main stresses acting upon the formation at the wellbore wall. This pressure is variable as the friction factors and fluid density changes with variations in the drilling process (i.e. pump rates, ROP, solid's loading, hole size…). This results in a continuous change of stresses around the wellbore during drilling that lead to deformation or even formation failure. This phenomenon has been widely documented 3,4 and at the wellbore wall resulting in the generation of a zone that has changed from an elastic to a plastic condition. The extent of this zone can be determined using the equations included in appendix 1.
This change also means a change in formation petrophysical properties throughout its width and length as it is illustrated in figure 1. In practical terms the two mechanisms (filtration and deformation) occur almost "simultaneously" during drilling and result in a distinctive zone with very different properties to those of the unaltered sandstone reservoir rock. In contrast with what is presented in petroleum engineering literature this damaged zone is not only invaded reservoir rock but deformed and invaded reservoir rock.
There is a significant difference between both terms as the latter implies mechanically unaltered rock whose matrix has been invaded by solids and mud filtrate, the former implies rock invaded in a similar manner but that has changed its initial properties as a result of the stresses in the near wellbore area. These changes in the rock mechanical state are induced by changing stress as initially described, a plastically deformed rock such as the one described above leads to a two (2) permeabilities zone as reported in the literature 5.
