Solvent / Acid Blend Provides Economic Single Step Matrix Acidizing Success for Fines and Organic Damage Removal in Sandstone Reservoirs: A Niger - Delta Case Study

Abstract

Abstract The formation of petroleum residue and deposits is a recurrent problem in low API gravity or heavy oil reservoirs and reservoirs with significant pressure and temperature fluctuations. It is also a challenge in fields where recent well interventions has pushed pipe dope into the perforations resulting in severe loss of native formation permeability and productivity. This paper discusses the application of a unique solvent acid dispersion that offers one step stimulation of reservoirs damaged with both plugging solids and organic deposits. Conventional acid systems including the numerous patented HF - HCl and organic acid blends dissolve the plugging solids and cementitious materials but cannot stimulate sandstone reservoirs damaged with organic deposits or a combination of plugging solids, fines migration and organic deposits. In fact, the formation of acid induced sludge is common. However, this acid - solvent blend dissolves asphaltene and paraffin deposits, prevents the formation of wax crystals and removes organic residue and inorganic materials simultaneously. A special surfactant train acts synergistically with the acid blend, prevents acid induced sludge and leaves the rock surfaces strongly water wet. Overall treatment efficiency was increased especially in gravel packed intervals by using a rotating jetting tool although success has also been recorded using the bullheading option. Recent experience has shown the acid blend is particularly suitable as a spearhead for injection wells. Optimum solvent loading for effective stimulation is between (10 – 20) percent depending on reservoir temperature, suspected damage mechanism and the severity of relative permeability effects. Unique experience detailing standard lab / field practice and exceptional results for several wells in the Niger Delta is the focus of this paper. Wells treated with this unique blend either as spearhead for other specialty acid systems or as a one step treatment show dramatic and sustained productivity increases of between (50 – 150) % relative to initial well data. The system has been proven to be successful in most types of completion architecture. Key technical and economic performance indicators including skin factors, production rates, specific productivity indices, payback time and treatment cost indicate this low viscosity acid blend is both technically proficient and cost effective. Introduction and Justification The in-situ remediation of damage caused by organic material deposited in the near wellbore region from well construction and intervention operations i.e. from POBM's, pipe dope, e.t.c. And from the producing system itself (waxes, resins, and asphaltenes) has historically been tackled using either one or a combination of the following measures: Hot oil - water treatments, Hot water - solvent combination, Solvent - surfactant - dispersant trains and crystal modifiers.1 The use of hot oil / water treatments is useful in the treatment of surface equipment clogged up with wax. Though these measures have attained a given degree of success even in reservoir stimulation applications, their continued usage has been proven to concentrate higher melting point carbon molecules in existing deposits, hereby making the resulting deposits more resistant to further heat treatments. This method has also been shown to cause significant formation damage by the way of negative relative permeability effects. Solvents and solvent / surfactant /dispersant trains are excellent means of organic damage removal in the reservoir. However, the extent of their organic carrying abilities is limited by temperatures below the cloud point of the organic material / solvent combination. Large quantities of solvents are typically required i.e., 100 lbs. of xylene will just dissolve 6 lbs. of C36 paraffin at 100°F.2 However, the use of the proper combination of chemicals (determined by proper lab tests) can reduce treatment volumes and costs considerably. The use of crystal modifiers or pour point depressants provide a more effective means of preventing deposits in the reservoir but are usually cost prohibitive.