In certain geological environments, hydrocarbon and mineral production can cause collapsing or subsidence of the local geostructure. Significant subsidence causes casing failure, either in the producing zone or in the overburden. Subsidence can be transmitted through the overburden to the surface causing major problems such as rig collapse or pipeline failure. Early detection of a subsidence problem is important so that measures can be taken to prevent the loss of the well or a potential blowout situation.
At the request of several operators, a technique has been developed for detecting small shifts in the formation or casing which indicate the onset of a subsidence problem. A pilot program using existing wireline tools (Fig. 1) was started in November 1990 on a well in the Gulf of Mexico. The technique and results of this project are presented in this paper.
This technique uses existing downhole wireline tools configured to provide high-resolution measurements. Core bullets (with no retaining wires), tagged with a very low-level, short half-life radioactive isotope (5 microC of cobalt 60,5.2-year half-life), are shot into the formation at regular intervals. After casing is set, a base log is run to determine the precise location of the casing collars and radioactive markers prior to production. Once production has begun monitor logs are run at regular time intervals to detect movement of the collars or markers. Any shift in the locations of the collars or gamma ray pips indicates casing deformation or formation subsidence. The analysis technique relies on detector spacing, rather than cable motion. Subsidence results are presented in graphical form as well as tabular listings.