Acoustic Surveillance of Production Impairment With Real-Time Completion Monitoring

Abstract

Abstract Deepwater production is challenged by well underperformance problems that are hard to diagnose early on and expensive to deal with later. Problems are amplified by reliance on few complex wells with sophisticated sand control media. New downhole data is required for better understanding and prevention of completion and formation damage. We introduce Real-Time Completion Monitoring (RTCM), a new non-intrusive surveillance method for identifying impairment in sand-screened completions that utilizes acoustic signals sent via the fluid column. These signals are carried by tube waves that move borehole fluid back and forth radially across the completion layers. Such tube waves are capable of "instant" testing of the presence or absence of fluid communication across the completion and are sensitive to changes occurring in sand screens, gravel sand, perforations, and possibly reservoir. The part of the completion that has different impairment from its neighbors will carry tube waves with modified signatures (velocity, attenuation) and also would produce a reflection from the boundary where impairment changes. The method relies on permanent acoustic sensors performing acoustic soundings at the start of production and then repeating these measurements during the life of the well. Thus, it could be thought of as "miniaturized" 4D seismic and "permanent log" in an individual wellbore. In the meantime repeated conventional wireline measurements can be performed to assess the completion permeability. Motivation Completions lie at the heart of deepwater production and present a large portion of the overall well cost. Great multidisciplinary effort is invested upfront to design them right. This contrasts with the production stage where little information is available to detect problems, optimize the inflow and prevent expensive workovers. Incomplete gravel packing, development of "hot spots" in screens, destabilization of the annular pack, fines migration, sand screen plugging, near-wellbore damage, crossflow, differential depletion, compartmentalization, compaction represent a typical list of challenges that are extremely difficult to decipher based on just several permanent pressure and temperature gauges. 1,2 Many problems can be identified by production logging, but it is costly and not in real time. Let us take the problem of underperforming wells in the Gulf of Mexico1,2. "Well performance" absorbs large-scale reservoir issues such as compartmentalization as well as changes in local well skin with time that further comprises of completion, perforations and near-wellbore effects. Therefore multiple explanations can be given to the problem. Apparent compartmentalization and ubiquitous U-shaped boundaries can be one answer on a "reservoir" scale. Yet those boundaries are rarely confirmed by 4D seismic or other data. Shale draping is an alternative reservoir-scale scenario that can lead to well underperformance. Another wellbore-scale explanation suggests that well productivity declines with time due to loss of so called "kh" where k and h are reservoir permeability and thickness respectively. The differential depletion model (Phil Fair and Fritz Rambow, personal communication) argues that this loss occurs mainly due to reduction in producing thickness although the exact mechanisms of flow impairment are still debated. Reduction in permeability is another alternative explanation offered by Pourciau2 although the amount of this reduction (85–90%) is not consistent with laboratory measurements. Existing sparse data from wells can support any of these scenarios confirming that the problem is underconstrained. In the context of deepwater completions there is an additional emphasis on sand control because it is believed that "Managing produced sand, as we understand it, is generally a costly and mostly unworkable solution for the Gulf of Mexico, but can work well in other places where there is some grain-to-grain cementation present" (Scott Lester of Shell, Sand Control3).