--> Abstract: Dolomite and Other Mineral Precipitation Associated with Steamflooding Heavy Oils in the Giant Wilmington Field, California, by M. P. Dempsey and D. E. Eby; #90987 (1993).

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DEMPSEY, MICHAEL P., Union Pacific Resources Co., Wilmington, CA; * DAVID E. EBY, Eby Petrography and Consulting Inc., Englewood, CO

ABSTRACT: Dolomite and Other Mineral Precipitation Associated with Steamflooding Heavy Oils in the Giant Wilmington Field, California

Significant dolomite and other mineral precipitation has occurred near and at the wellbore of producing wells within steamflooded portions of Wilmington field, Los Angeles Co., CA. Thermal modification of the 12 degree API gravity heavy oils from the Tar Zone in the Pliocene Repeto Formation results in the production of 14 degree API gravity crudes as well as increased carbon dioxide and hydrogen sulfide. Following a period of significantly increased oil production after steaming commenced, many producing wells experienced dramatic production declines.

Coring and wellbore bailing during redrill operations have recovered significant mineral "cements" that apparently result from major dissolution of silicate grain matrix in the steamed zones, displacement of previous waterflood waters that originated from adjoining modern ocean waters,

and a major pressure/temperature drop at the wellbore. These factors have resulted in a zonation of minerals that precipitated rapidly in the near-wellbore reservoir, in the quartz gravel pack and between the casing and tubing. Cementation by coarsely crystalline dolomite, other Mg-, Mn-, Ca- and Na-carbonates, and a variety of silicate minerals is apparently complete enough to decrease fluid entry into the wellbore. This "artificial diagenesis" can be seen as intergranular cements within the previous unconsolidated sands, as new "fracture" fillings within the unlithified formation muds or as amorphous precipitated masses. Coring of a low-angle deviated redrill well shows that the formation plugging is significantly reduced a short distance away from the wellbore. Effective strategies have been developed to minimize the effects of steamflood diagenesis.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.