Society of Petroleum Engineers Distinguished Lecturer 2003-04 Lecture Season
Oilfield Subsidence: The Five W's (What? Why? 2xWhere? & What!)
Abstract
Oilfield subsidence, and the associated reservoir compaction that causes it, has occurred around the world from the North Sea to South America and elsewhere. This presentation will highlight the five W's of compaction and subsidence: What it is; Why it occurs; Where it has been seen; Where we can expect to see it in the future; and What do we do about it(!). Subsidence is simply the movement of surface strata in response to a loss of underground support often associated with reservoir compaction due to hydrocarbon withdrawal. Why compaction and subsidence occur varies from the simple response of strata to a pore pressure decline to a complex interaction of pressure, temperature, and fluid saturations all influenced by underburden, reservoir, and overburden geometry and properties. Subsidence at the Wilmington and Ekofisk oil fields are two of the most widely recognized examples due both to the magnitude of subsidence as well as the cost of remediation. However, while lesser known, subsidence is a challenge for a number of other oilfields. In Venezuela, subsidence due to reservoir depletion has led to severe flooding along the coast of Lake Maracaibo. In the Netherlands, subsidence at the large Groningen gas field, though only on the order of tens of centimeters, poses significant challenges since large portions of the Netherlands are at or below sea level and protected by dikes. New fields, particularly weak sand reservoirs, also have a subsidence potential, and as more and more of these types of fields are exploited, compaction and subsidence issues will become important here. Finally, reservoir compaction and subsidence have many impacts, challenges, solutions, and even benefits. Seabed subsidence at the Ekofisk field, for example, has had a well known effect by reducing platform airgap and resulting in the jacking of platforms in 1987, the barrier placement in 1989, and the Ekofisk II redevelopment in 1998. Likewise, subsidence has led to significant pipeline concerns due to excess compressional or tensional strain. Reservoir compaction itself has led to numerous casing deformations and poses a notable challenge for well completion. However, reservoir compaction also provides significant drive energy and greatly contributes to increased production and reserves.
Biography
Neal B. Nagel is currently a principal geomechanics engineer with ConocoPhillips in Houston. Nagel joined Phillips Petroleum in 1989 after working as an assistant professor and visiting assistant professor at New Mexico Tech and Southern Illinois U., respectively. With Phillips, Nagel worked in the Production Technology Branch until transferring to the Stavanger office in 1993. From 1993 to 1999, he served as the subsidence engineer for Phillips Norway. From 1999 to 2002, Nagel maintained responsibility for Ekofisk-area geomechanics issues, particularly in the area of geomechanics monitoring. In addition, he served as the team leader for the GeoMechTeam, a group of engineers and scientists addressing geomechanics and completion issues for Phillips' worldwide operations. With the merger of Phillips and Conoco in 2002, Nagel transferred to the Production Assurance and Optimization group within Upstream Technology of ConocoPhillips and works on production geomechanics issues such as sand prediction and wellbore stability as well as maintaining involvement in global compaction and subsidence challenges for the company.
Nagel holds PhD, MSc, and BS degrees in mining engineering from the U. of Missouri-Rolla. He has been an SPE section officer, served on the Well Completions committee for the annual technical conference, authored numerous SPE and technical papers, and is a member of SPE, ARMA, and ISRM.