Mineralogy and Origin of Well-Bore Scales in an Active Steamflood: Tar Zone, Fault Block IIA, Wilmington Field, California

David K. Davies, Richard K. Vessell, Robert Y. Fu, Julius J. Mondragon, David A. Stepp, and Omar J. Aguey

Precipitation of well-bore scale is a significant problem in the active steam drive project in the Tar Zone of Fault Block IIA, Wilmington Field. This present study is part of the US Department of Energy Class III program to improve oil recovery in slope and basin reservoirs. The aim of the study is to design the least expensive treatment to minimize scale precipitation. As the first step in this program, it is necessary to precisely identify the mineralogy and origin of the scales.

Three different families of well-bore scale occur. 1) Carbonate scale, dolomite, high and low Mg-calcite; Mg content increases with decreasing well bore temperature. Calcium originates from connate waters: Mg originates from waterflood seawater: Mn and some Mg originate from framework grains. Routine acidizing with 15% HCl readily removes this scale. 2) Sulfate scale, barite and anhydrite, is not common. Sulfates can be interlayered with carbonates and were precipitated at relatively low temperatures. Sulfates have small attachment areas and active well-bore brushing removes them with or without acid treatment. 3) Hydrous Magnesium Silicate scale occurs sparingly. The actual mineral form is difficult to identify because of pooriy developed crystal structure. This mineral represents precipitation at the lowest temperature of all scales in the field. It is non-soluble in acid and must be mechanically removed.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California