Abstract
During the process of water injection, due to solid particle deposition and foreign liquid intrusion, the formation near the wellbore was contaminated and blocked. As a result, water injection rate reduced and failed to meet the injection requirements. In order to improve water injection rate and improve oil recovery of offshore oilfields, hydraulic injection tests were carried out in controlled laboratory conditions. In general, the formation of complex fracture patterns is an ideal outcome of the hydraulic fracturing stimulation seeks to achieve. In situ stress condition is an inherited geological condition one can only adopt to. By comparing test results of different experiments that had varied stress and hydraulic injection conditions imposed, we can investigate their impact on the fracture patterns created. This paper presents laboratory evidences to support that if the hydraulic injection condition is managed properly, a complex fracture pattern is possible even under a high in situ stress anisotropy. Even if the in situ stress condition has a large anisotropy, proper hydraulic stimulation operations can still cause complex fracture patterns and thus provide good stimulation efficiency.
Highlights
In offshore oilfield, many mature injection wells are faced with challenges of being severely blocked and face difficulties reaching allocated injection rate targets
Hydraulic dilation uses controlled high-pressure injection to alter the microstructures of a rock material, forming a zone of increased porosity and localized microtensile cracks
A smaller difference between the prescribed principal stresses could form discrete fracture paths, while a larger difference caused a complex fracture geometry
Summary
Many mature injection wells are faced with challenges of being severely blocked and face difficulties reaching allocated injection rate targets. Based on laboratory test results and field test data, Yuan [3] showed that these microtensile cracks are often arranged into echelon arrays They do not connect with each other to form a continuous fracture path, but are dispersed separately in the otherwise continuous rock medium. A smaller difference between the prescribed principal stresses could form discrete fracture paths, while a larger difference (but with properly managed hydraulic injection conditions) caused a complex fracture geometry (i.e., numerous microcracks were observed in the samples). This is contrary to the traditional thought about impact of in situ stress anisotropy on fracture complexities. A discussion is given about the underlying mechanisms, their relevance to the current knowledge on fracture complexities, and how the laboratory test will guide the field operations for oil/gas production
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