EFFECT OF THE PLASTIC ZONE CHARACTERISTICS ON SAND PRODUCTION

Sand production is a generally undesirable process where solid sand particles are produced with oil or gas. Weak formations are usually prone to sand production problems as the stress states under production conditions easily exceed the mechanical strength of the reservoir rock. The stress difference is particularly acute near holes, such as a perforation tunnel. Thus the plastic zone of failed material is created around perforations. The formation of the plastic zone around a hole can be related to the mechanical characteristics of the weak sandstones. The analysis of the plastic zone properties is essential in addressing sand prediction problems in weak sandstones as sand is produced from the failed material in the plastic zone and the existing knowledge on the plastic zone properties is limited and contradicting. This research analytically and experimentally investigates the role of the plastic zone permeability and porosity on the prediction of sand production. Large scale sand production experiments were conducted using a customized High-Pressure Consolidation System. The existing sand prediction models were upgraded with new information on the plastic zone permeability and porosity distributions to achieve a semi-analytical prediction model for the sand production from a hollow cylinder weak artificial sandstone under radial flow. The obtained model was validated with the sand data from a weak sandstone reservoir. The research begins with examining two different weak artificial sandstone preparation techniques. Sandstone strength is associated with bond strength as these sandstones were made of strong particle and weak cement. Weak sandstone behaviour was investigated using one-dimensional compression tests, triaxial compression tests, particle characterization, and compositional analysis tools. The results show that weak sandstone behaviour can be characterized as an initial contraction of the sample before the peak stress followed by large dilation. The appropriate artificial sandstone preparation was chosen based on analysis for the further sand production experiments. Moreover, mechanical properties such as unconfined compressive strength, cohesive strength and friction angle were retrieved from the mechanical tests as input parameters for the further analytical study of the plastic zone permeability. Large scale sand production experiments were then conducted at different stress-fluid pressure conditions. The results were used to investigate plastic zone characteristics such as permeability and porosity distribution. The existing sand prediction models based on viscoplastic granular flow and hydromechanical erosion were modified to include the obtained experimental and analytical results on the plastic zone properties to develop a semi-analytical sand prediction model. The results show that specific stress-fluid pressure conditions may create the plastic zone around a hole, which has a decreased permeability compared to the intact zone. In addition, the sand production process was characterized by the permeability increase in the plastic zone from the decreased value. Plastic zone permeability and porosity distribution were incorporated into the existing sand prediction models and the models were validated with sand production experiments. The obtained model was used to predict sanding rate data from oil-producing wells. The results indicate the significance of the plastic zone porosity and permeability distribution in the sand production prediction, and, the model performance is positive for further development and improvement in the laboratory and the real applications. The applied fluid and rock stresses along with the initial permeability of the sample and fluid viscosity govern the plastic zone properties change. The novelty of the research consists of the detailed study of and proposed model for the plastic zone permeability and porosity change during the sand production process