Drilling and Acidizing Sandstone Stringers-Sludge Characterization and Acid Interactions with Rock Minerals: Integrated Geochemical and Engineering Techniques

摘要

Understanding the sludge types and causes is essential to preventing various drilling issues,especially during the drilling of heterogeneous sandstone reservoirs where the permeability is variable.Acidizing these sandstone reservoirs is challenging because of their inconsistent mineralogical types and contents resulting in different reaction behaviors with drilling fluids and acidizing recipes.The current study uses new techniques to assess sludge types.The sludge characterization revealed two types that are different in appearance and composition.The first type is solid,and is characterized by black to brownish colors,vitreous luster,and conchoidal fractures.This is described as bituminous sediment.The pyrolysis results showed that the sludge samples are composed of a mixture of diesel and solid bitumen or coal fragments.Therefore,the samples were investigated with organic microscopy techniques.The coal particles contain vitrinite,tellovitrinite,resinite,and solid bitumen.Part of the samples were treated with xylene to extract the hydrocarbon portion.The result revealed that the sludge sample is composed of 70 wt% organic matter and the remaining is inorganic.Calcite constitutes 82 wt% of the inorganic components,leaving 18 wt% for silicate minerals.Most of this type 1 sludge was dissolved by xylene.This indicates that they represent solid bitumen,which absorbs oil-based drilling fluids.This results in destabilizing the substrate,which causes the bit to abort drilling.Therefore,water-based drilling is recommended in this case.The type 2 sludge is composed of fin e-grained sediments saturated with liquid hydrocarbon-mostly oil.A total of five sludge samples were characterized by high resolution pyrolysis and revealed variable saturates,aromatic,resin,and asphaltene percentages corresponding to different degrees of oil and drilling fluid incompatibilities.The type 2 sludge sample showed a slight solubility in mutual solvents and xylene at 160 °F,i.e.,5 wt% to 19 wt%.The dissolution of the type 2 sludge sample in formic acid was higher than acetic acid when mixed with either ether-based mutual solvent or microemulsion solvent.The highest solubil-ity values were obtained with mixtures of 5 wt% hydrochloric(HCl)acid mixed with solvents,showing solubilities between 79 wt% to 88 wt%.Coreflood experiments utilizing HC1 acid and formic acid mixtures showed a slight reduction in permeability-i.e.,20%-using highly permeable sandstone core plugs.Injection of HC1 acid and formic acid mixtures or 10 wt% formic acid in the presence of 0.4 vol% clay stabilizer showed a reduction in the permeability by 5% to 13%.Although,the coreflood experiments showed a slight reduction in permeability,the computed tomography(CT)scan results obtained showed a significant increase in rock density along the core length for core plug Nos.2 and 3,indicating the occurrence of chemical reactions with rock minerals that had an insignificant impact on the core permeability.