Drilling Fluid Additives for Wellbore Strengthening and Reservoir Protection

The purpose of the study is to better understand situations related to lost circulation of drilling fluid, when drilling wells for the production of oil and gas or geothermal wells. The knowledge will be used to optimize additives for drilling fluids so that one can strengthen the wellbore during drilling and prevent the reservoir formation from being damaged during the drilling operation. In order to be able to optimize such additives it is important to identify mechanisms that effectively contribute towards creating temporary seals on permeable or fractured formations. Then the conditions that the drilling fluid is exposed to in the form of, for example, pressure, temperature and circulation in the well must be substantially recreated to identify which factors contribute to the desired functionality to the greatest extent. In the first phase of the project, the current API-13 test procedures have been used to study how drilling fluids with various additives seal permeable formations of different kind. At the same time, new test methods have been developed to investigate how new information can be gathered by extending the testing described by API-13. Base on the research that has been conducted, seven research papers have been published. Seven of these have been published in scientific journals, and two have been presented at industry conferences. The research papers cover topics of new testing methodology and application of newly developed products for enhanced wellbore stability and reduced formation damage. The overall results show consistency between the results from the field applications and the predictions made based on the results of the extended test methods. In its entirety, the results of the research enable the function of drilling fluids to be improved both in relation to strengthening the wellbore during drilling and to avoid formation damage when drilling reservoir formations.

Advances in testing methodology have been published so that benefits may be shared with Standard Norge /CEN/ISO/API. • Improved products and drilling practices leading to faster and safer drilling, e.g.: o Exploration wells may be drilled with higher safety margins on mud weights, thereby limiting occurrence of blow-out situations o Lower fluid losses enabling reduced cost and environmental footprint o Reduced risk of cavings in fragile formations o Better pressure isolation to reduce risk of differential sticking • Improved production through reducing damage to reservoir formations and enabling extended reach drilling in reservoirs o Improved return permeability will lead to more effective reservoir drainage and hence improved net present value of well o Extended reach drilling will enable better reservoir management and reaching further towards the boundaries for the reservoir for either injector or producer wells • Benefits will also be applicable for geothermal wells

Drilling fluids have many functions including removing cuttings from the wellbore, cooling and lubricating the bit as well as controlling formation pressures, sealing permeable formations and maintaining wellbore stability and well control. "Lost circulation" is when drilling fluid is lost to the formation and the ability to control and stabilise the well is partially or fully lost. Situations related to lost circulation are amongst the most costly issues related to application of drilling fluids and may also be detrimental to well control and safety. Current industry standards for testing of drilling fluids, such as API-13, have limited procedures for testing and classifying drilling fluids and drilling fluid additives when it comes to lost circulation, wellbore strengthening and reservoar formation protection. The objective of the project is to better understand lost circulation situations and lost circulation materials (LCM) and design improved products for better functionality from a cost/benefit, safety and environmental perspective. In order to develop more efficient LCM it is necessary to understand the sealing mechanisms of existing LCM, understand the industry standard testing regimes and potentially identify how such testing regimes may be extended or improved to better replicate downhole conditions. Also, LCM materials must be designed to effectively work together with solids control systems, downhole tools/bottom hole assemblys and other parts of the circulation system, as well as the other elements of the drilling fluid. The latter factors are particularly important for LCM designed to be a part of the active system to prevent and limit occurrence of losses and to prevent drilling fluid induced formation damage when drilling reservoir sections.