Solids Loading Assessment for Produced Water Reinjection in a Carbonate Reservoir
Authors: Chen Peng , Willingham Thomas , Al Sowaidi Alunood , Stojkovic Dragan , Brown James 
Published: November 2018

In the oil industry, oil and gas are usually accompanied with water when they are produced from the subsurface. How to tackle water is one of the major concerns for the field development, especially as fields mature and water production increases. Produced water reinjection (PWRI) has been considered an environmentally friendly way to handle large amounts of waste fluid, though it needs to be carefully designed. In this paper we present a lab study conducted to determine the water specification requirements for reinjecting produced water back into the subject carbonate reservoirs.

The objective of this study is to assess the required produced water quality to maintain matrix injection into the targeted reservoirs. The assessment includes (1) evaluation of the inorganic scaling potential of water sources (fluid compatibility), (2) core flood tests to quantify the impact of various oil content concentrations of produced water on reservoir performance, and (3) a solids loading core flood test to evaluate the injectivity impact of different filtration sizes and different suspended solid concentrations in the produced water. While the previously published paper (Chen et al., 2017) already addresses the scaling and oil content assessments, this paper will present the details of the solids loading core flood test.

Produced water (PW) collected from the field was utilized in all stages of this study. Analysis of the composition of the suspended solids in the collected produced water revealed a large amount of iron in the PW’s suspended solids, most likely a corrosion product from the long-distance pipeline between the subject field and the current water treatment and separation facilities. Consequently, the collected produced water’s particle size distribution is inadequate to represent the future reinjected produced water which will come from artificial island wells without going through the pipeline. To replicate the anticipated particle size distribution, filtered produced water was mixed with synthetic solid micro particles according to the particle size distribution measured at the well head and the solids loading specification from the skimmer design to mimic the ‘outlet water’ from the skimmer. The skimmer ‘outlet water’ was then filtered to different sizes, starting with 2μm and relaxing the filtration requirements with each step. To replicate oil carryover, 300 ppm of the field’s oil was added to the sequential filtration stages of the skimmer ‘outlet water’ and was flowed through a preserved core plug of the field’s dominant rock type.

Coreflood results suggest that for particle concentrations which represent the solids loading coming from the designed skimmer (TSS=33mg/L), a surface/external filter cake may form with no significant particle penetration into the rock matrix when filtration size is larger than 2µm. More specifically, particles smaller than 2µm did not contribute to the permeability decline, and most of the permeability decline was caused by a filter cake composed of particles in the 5-10µm range. Particles larger than 10µm do not have a significant effect on the permeability decline, most likely due to their low concentration.