During drilling and completion the primary mechanisms of near-wellbore formation damage include pore-throat constriction, water blocking, plugging with drill solids and mud products, and loading of the reservoir with drilling or completion fluids. Among these mechanisms, some of the most severe ones encountered in elastic reservoir applications are the pore-throat constriction due to clay swelling, and water blocking resulting in a reduction in the relative permeability to hydrocarbons.
A novel matrix stimulation concept which involves the application of intense heat for the treatment of water-blockage and clay-related formation damage in water-sensitive formation is presented in this paper. Bench-scale heating tests were carried out on water-sensitive sandstone cores to determine the effect of heat on effective permeability, fluid saturation, and mineralogy (i.e., degradation of in-situ minerals). Results indicated that heat treatment at 600 °C can improve air permeability of a damaged core by about 51% bove the initial permeability. Dramatic permeability increases f 764% and 988% above the initial reservoir permeability occurred at 800 °C for the cores taken from the gas- and oil-bearing formation, respectively respectively.
Formation damage can occur at any time during a well's history from the initial drilling and completion of the wellbore through the depletion of the reservoir during production. Operations such as drilling, completion, workovers, and stimulation, which expose the formation to a foreign fluid, may cause formation damage because of adverse wellbore-fluid to formation interactions. Such damage is usually severe in horizontal wells, because of the longer exposure of the wellbore to the offending fluids. 1 During the drilling and completion phases, the primary mechanisms of near-wellbore formation damage can be explained by the following factors:
pore-throat constriction, caused either by clay swelling
due to incompatible fluids or by clay migration,
water blocking due to reduction in relative permeability to hydrocarbon,
plugging with drill solids and mud products, and
loading of the reservoir with drilling or completion fluids.
Clay-related formation damage during drilling and completion has long been identified to be a major problem. Preventive measures to stabilize clay swelling and migration, mostly consisting of the use of various chemicals (e.g., KCI) in the drilling or completion fluid, have been discussed in the literature.2–7 However, prevention of clay damage is not always possible or effective, and curative measures may then become necessary. Several curative methods have been attempted and presented in me literature.8–15
One approach is to bypass the near-wellbore damage using hydraulic fracturing. This technique is very effective in sandstone formations and in vertical wells. However, there are situations where hydraulic fracturing is not desirable (e.g., in water- or gas-flooding situations, zones containing active bottom water or gas caps) or nor economical (e.g., in some horizontal wells).
Another approach is to stimulate the near-wellbore region using acids, which dissolve either the clay minerals themselves (HF acid) or the surrounding formation rock (HCl and HF acids).