During the past decade, waste injection (WI) technology, also known as cuttings re-injection (CRI) technology, has been gradually accepted as an environmentally-friendly and cost-effective ultimate disposal method for drilling-related solids and liquid materials. In waste-injection operations, waste slurry is usually intermittently batch injected into appropriately selected subsurface formations. These slurry injections cause injection pressure and in-situ stress to gradually increase as more and more injected solid tends to accumulate in the near-wellbore fracture domain area. Moreover, solids in the leftover slurry can settle out and plug the wellbore during shut-in periods. These issues challenge injection equipments, wellbore integrity, waste domain containment and disposal capacity, and may hinder or terminate injection operations unexpectedly. Therefore, slurry must be properly overflushed from the wellbore and near-wellbore fracture area effectively and immediately. Effective execution and engineering analysis of post-slurry overflushing have become keys for successfully maintaining critical injection assets. A vigilant monitoring of surface, downhole tubing and annulus pressures makes these analyses possible in a timely and proactive manner.
In this paper, post-slurry overflushing with viscous pill and solids-free seawater is numerically modeled for analyzing efficiency of the overflushing and the effects of volume and rheology of the pill on the overflushing in a waste injection well. Moreover, the continuous pressure monitoring in two waste-injection projects in two offshore oilfields provides long-term downhole pressure data. These data were collected and analyzed for instantaneous shut-in pressure (ISIP) after each slurry injection and each post-slurry overflushing, and he effect on in-situ minimum stress and injection pressure was analyzed by comparing ISIP changes before and after overflushing. The studies on ISIP changes confirm the results of the numerical modeling in this paper.
Field studies and modeling results also show that overflushing with the appropriate rheologically-designed viscous pill/seawater sequence can decrease long-term injection pressure buildup rate, avoid wellbore/fracturing plugging and extend ultimate disposal capacity.