Understanding the sludge types and causes is essential to prevent various drilling issues especially during drilling of heterogeneous sandstone reservoirs where the permeability is variable. Moreover, acidizing these sandstone reservoir is challenging because of their inconsistent mineralogical types and contents resulting in different reactions 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 characterizes by black to brownish colors, vitreous luster, and conchoidal fracture. This is described as bituminous sediments. The pyrolysis results elucidated that the sludge samples are composed of mixture of diesel and sloid bitumen/coal fragments. Therefore, the samples were investigated with organic microscopy techniques. The coal particles contain vitrinite, tellovitrinite and resinite and solid bitumen. Part of the samples were treated with Xylene to extract the hydrocarbon proportion. The result revealed that the sludge sample is composed of 70 wt% organic matter and remaining is inorganic. Calcite constitutes 82 wt% of the inorganic components leaving 18 wt% for silicate minerals. Since the majority of this sludge type 1 was dissolved by Xylene, this indicate that they represent solid bitumen which absorb oil based mud. This results in destabilizing the substrate which cause the bit to abort drilling. Therefore, water-based drilling is recommended in this case. On the other hand, the second sludge type is composed of fine 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 degree of oil and drilling fluid incompatibilities.

Sludge samples type 2 showed slight solubility in mutual solvents and xylene (i.e., 5-19 wt%) at 160°F. The dissolution of sludge samples type 2 in formic acid was higher than acetic acid when mixed with either ether-based mutual solvent or microemulsion solvent. The highest solubility values were obtained with mixtures of 5 wt% HCl mixed with solvents, showing solubilities between 79-88 wt%.

Coreflood experiments utilizing HCl/formic acid mixtures showed slight reduction in permeability (i.e. 20%) using highly permeable sandstone core plugs. Injection of HCl/Formic acid mixtures or 10 wt% formic acid in the presence of 0.4 vol% clay stabilizer showed reduction in perbeability by 5-13%. Althgouh, the coreflood experiments showed slight reduction in permeability, the CT scan results obtained showed significant increase in rock density along the core length for core plug No. 2 and 3, indicating occurance of chemical reactions with rock minerals that had insignificant impact on the core permeability.

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