Scale Mineralogy and in-situ Silica Diagenesis in the Diatomite Steam Drive and Cyclic Steam Wells, Belridge Field, Kern County, California – Interpretation of Scale Composition, Texture, and Diagenesis

Bloeser, Bonnie; Nelis, Mary; and Hume, Ann
[email protected]

Water and steam, injected in the opal-A diatomite reservoir for pressure maintenance and secondary recovery enhancement, has led to the precipitation of scale ranging up to 12mm thick on tubing, pumps, and flow lines. Highly microporous 4mm argillaceous porcelanite (opal-CT/clay) balls concentrated at the group separator test facility suggests that silica diagenesis occurs downhole. Scale inhibits fluid flow in the near-wellbore environment by reducing pore throats in the reservoir rock, by restricting flow at the perforations, and by pump failure. We hypothesize that silica diagenesis, from opal-A to opal-CT and perhaps from opal-CT to microcrystalline quartz, is occurring in the near wellbore environment based on laboratory analysis of solids found in the wellbore and in the test facility.

Scale samples from twelve wells and eight samples from the group separator testing facility, in the diatomite steam drive and cyclic steam recovery areas of the Belridge Field, were analyzed by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and thin-section petrography. Results reveal complex scale minerals, fabrics, and textures from the mixing of the slightly argillaceous opal-A diatomite reservoir, injected and formation fluids, and pipe corrosion products. Minerals include an array of calcium, iron, and manganese carbonates, magnetite, halite, zeolites (mordenite), clays, and silica diagenetic products, often exhibiting zoning and convolute laminae, and often mixed with residual asphaltenes. Bitumen is often interlayered with cryptocrystalline calcite layers that exhibit botryoidal, fibrous, and colloform growth fabrics. Growth zones are often defined by traces or clumps of impurities.

Solids sample analysis and interpretation from the group separator indicate that the high temperatures of the cyclic steam process (wellhead temperatures average 329°C/625°F and downhole temperatures at 1200’/366m average 232°C/450°F) are sufficient to drive the silica diagenesis reaction from biogenic opal-A to microcrystalline opal-CT. XRD cristobalite peaks with a d-spacing average of 4.05 and a crystallinity index (QCI) of 6.1 (i.e. non-detrital quartz), plus the observation of opal-CT lepispheres confirms that silica diagenesis occurs downhole. We infer that the transformation occurred due to elevated temperatures in the cyclic steam recovery process and may also occur in the steam drive areas where temperatures are sufficient to drive the reaction to opal-CT.

Solid samples collected from the cyclic steam group separator site consist of dark gray, non-indurated, 3-7mm spheres in which X-ray diffractograms record significant cristobalite peaks (viz. 53% opal-CT and 23% illite). SEM confirms the presence of siliceous microfossil relics, moldic pores in the shape of diatoms, and opal-CT lepispheres support the interpretation that the samples are opal-A diatomite that has undergone silica diagenesis. It is surmised that the presence of microcrystalline quartz resulted from further diagenesis of opal-CT to quartz. Although the wells are completed in the opal-A diatomite reservoir, the presence of opal-CT was theorized due to the high temperatures in the reservoir. The origin of the opal-CT in the aggregate opal-CT/clay balls is thought to result from dissolution of the opal-A and recrystallization of opal-CT in the presence of elevated temperatures. The high concentration of clay in the opal-CT/clay balls results from fines mobilization from the reservoir into the wellbore. Platy clay particles mix with the opal-CT where together they form aggregate balls by physical agitation in the wellbore.

Wellbore scale and solids material identification is not only critical to remediation treatment but provides direct evidence of the chemical dissolution and recrystallization reactions that occur downhole, in the near wellbore reservoir environment, in flow lines, and at fluid treatment facilities. Knowledge gained from these analyses aids in understanding the chemical interactions between the opal-A reservoir rock, injected fluids and gas (steam), and silica diagenesis to provide solutions for improved flow into the wellbore, how to minimize particle aggregation, and improved scale control.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013