Abstract

This paper investigates the suspended particle retention rate and permeability damage in porous sandstone during produced water injection process. The data used in this study was obtained from one-dimensional constant rate injection tests in which both the effluent concentration and the pressure drop were recorded. The particle retention process was modeled as a filtration process using finite difference method. The intrinsic filtration and permeability reduction functions were determined by matching the overall measured effluent concentration and pressure drop data. Least square method was used as an optimization technique in the back analysis.

Introduction

Produced water re-injection process is considered as the most suitable and economical method to dispose the produced waste water with minimum environment impact. However, the loss of permeability and injectivity due to the particle retention within the formation around the injection wells is a common problem in this process.

Mettananda (2005) conducted 1-D injection tests with particle suspensions to study the particle retention and permeability reduction in sandstone specimens. In this paper, systematic methodologies are developed to analyze the results measured from these injection tests to determine the filtration and permeability reduction functions.

Particle suspension Injection experiments (Mettananda 2005)
Materials and fluids used

The sandstone specimens used in the study were recovered at depths of about 1750–1774 m from Camaal-30 Well of the Qishn formation, Masila block, Yemen. Specimens of 3.8 cm in diameter and 6.3 cm in length were trimmed from drilled cores. Fig. 1 shows a typical thin section of Qishn sandstone specimen. Results from mercury intrusion porosimetry tests show that the porosity values lie in a range of 0.17-0.23, and the median pore throat sizes are within 10–33 µm. Fig. 2 shows a typical pore throat size distribution of Qishn sandstone specimen.

Formation water present in oilfields is usually brine. Results of the chemical analysis of Qishn sandstone formation water showed that the main cations present are Na+ and Ca2+ while the main anions are Cl, SO42-and HCO3-. Based on the milliequivalents, the equivalent NaCl concentration was estimated to be 2646 mg/l. To be on the conservative side, 5 g/l NaCl brine was used in the flow experiments.

Silica fume was selected to be the type of particle used in the experiments. Silica fume is a commercially available admixture used in concrete material. It does not flocculate in NaCl solution and has a wide size range of 0.4 µm to 60 µm. Based on the pore throat size distribution of the core specimens (median pore throat sizes are 10–33 µm), the injected particles should have a size range between approximately 0.6 µm to 8 µm. Sedimentation method was used to achieve a good separation.

Test setup

In this study, a flexible wall permeameter (ASTM D 5084- 03) was used in the 1-D injection tests. In this method, the same confining pressure is applied to the specimen, both in radial and axial directions. A schematic of the permeameter used is shown in Fig. 3.

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