We present results of Real-Time Completion Monitoring (RTCM) with acoustic waves on a full-scale model of a cased deepwater well with sand-screened completion. First, we describe the results of active surveillance with a controlled source. Using permanent fiber-optic sensors and observing changes in tube-wave signatures we detect changes in permeability along the completion. We prove that such measurements can be conducted while the well is flowing and show that continuous surveillance also allows monitoring technological processes such as gravel packing. Then we describe the first results of passive listening that allows locating flowing perforations and speculate on the possibility of detecting flow velocity. Finally, we outline a possible path to implement RTCM technology.
Deepwater production increasingly relies on a few precious wells that are complex and expensive. Success is critically dependent on our ability to understand and manage these wells particularly at the sandface. These wells are filled with expensive “jewelry” like sand control and production allocation systems that aim at maximizing production and minimizing risk. While this smart equipment can mitigate many anticipated dangers, it can easily fail when something less expected happens. For example, repairing a sand control system failed due to plugging can cost US$30-40 million. Costs of lost production due to long-term well impairment can be much higher. Lower than expected production is often referred to as “well underperformance” (Wong et al, 2003) and can be caused by various impairments: a plugged sand screen, contaminated gravel sand, clogged perforations, damaged formation around the wellbore or larger-scale compartmentalization. While 4D seismic can address large-scale compartmentalization, it has no resolution to address near-well issues. Scarce downhole data from pressure and temperature gauges also cannot unambiguously characterize the impairment. This limits mitigation opportunities and prevents us from finding more effective drawdown strategies for high-rate high-ultimate- recovery deepwater wells. We strongly believe that geophysical surveillance in boreholes has a big role to play in identifying sources of well impairment and optimizing production. Here we describe one possible avenue - Real-Time Completion Monitoring (RTCM) – that utilizes acoustic signals in the fluid column to monitor changes in permeability along the completion. In essence, this is a miniaturized 4D seismic in a well. Bakulin et al (2008a,b) introduced the concept and presented modeling and initial experiments without gravel sand. Here we illustrate capabilities of acoustic surveillance through a series of full-scale laboratory tests with a more realistic gravel-packed completion, prove that measurements can be done while the well is flowing and outline capabilities of passive surveillance.
At low frequencies acoustic signals in a fluid-filled borehole are mainly carried by tube or Stoneley waves. Inside the fluid column the tube wave mainly consists of a piston-like motion. When fluid is compressed, it attempts to expand radially and pushes against the formation or casing. When the borehole wall is permeable, then the tube wave can move the fluid through and this leads to a slowdown in velocity and an increase in attenuation (Figure 1a). Currently these principles are employed in estimating near-wellbore permeability from an open-hole acoustic logging (Tang and Cheng, 2004).
