Using Crosswell Electromagnetics to Map Water Saturation and Formation Structure at Lost Hills

Abstract

In 1992, Chevron USA initiated a large waterflood program in the Belridge Diatomite at Lost Hills Field. Injection and production wells are hydraulically propped-fractured, and water is injected across a vertical interval of approximately 600 ft. The injected water is assumed to follow the hydraulic fractures and spread laterally outward. However, the extent of flooding, the saturation variations between individual layers, and the connection between waterflood patterns are poorly understood. During this same time period, crosswell electromagnetic technology was developed and tested in other fields in the San Joaquin Valley. This technology can provide an interwell resistivity distribution from wells separated by up to 1000 ft, and thus, may be suitable for mapping water saturation variation and reservoir structure. In this paper we describe the application of crosswell EM technology to several waterflood patterns at Lost Hills. The first cases involve surveys between closely spaced fiberglass-cased observation wells across and perpendicular to a hydraulic fracture. The interwell resistivity distribution identified the edge of the waterflooded region and mapped saturation variation between the layers that were affected by the flood. In another example, crosswell data that were collected parallel to hydraulic fracture planes show the connection of injection plumes and also significant saturation variation between layers. Introduction Producing oil and gas at the Lost Hills field is an ever-increasing challenge. Although over 2 billion barrels of oil is in place, oil production is difficult due to low matrix permeability and unusual formation properties in the diatomite. Operators are frequently plagued by issues of low production rates, ground subsidence and well instability. Chevron USA is testing a variety of technologies to improve both production and overall recovery. These include induced fracturing, and water, steam and CO2 floods. Most new technologies are tested in pilot studies. Considerable effort is expended to monitor the results of the pilot tests in order to track reservoir changes during production and injection, improve production and injection rates, better characterize existing resources, and determine the best alternatives for expanding the pilots and increasing oil recovery. As part of the waterflooding program, Chevron has established several pilot sites where a number of observation wells are drilled and new production, injection or monitoring technologies are tested. In two of these pilots, Chevron has installed two fiberglass-cased wells surrounding an injection well and has applied several logging technologies, as well as crosswell electromagnetics (EM) to evaluate the present status of the resource and to identify the progress of the ongoing waterflood. The crosswell EM technology is attractive because of its capability to map the interwell resistivity distribution. This, in turn, allows for convenient tracking of ongoing flooding operations as well as improved reservoir characterization. In this paper we will briefly describe the crosswell EM technology and illustrate its application to reservoir characterization using data collected at Lost Hills.

Application of a New Mechanistic Decline Curve Method to Kern County Oil Fields

“…Another example is the location of the main production in the Lost Hills field, which is in an upper member of the Miocene Monterey Formation known as the Belridge Diatomite. This reservoir is characterized by very low matrix permeability (0.01 to 10 md) and fair oil saturation (50%) 33 . This also will yield lower a o and b o values.…”

Pore Microstructure and Fluid Distribution in a Diatomaceous Reservoir

“…Crosswell electromagnetic survey results indicate that the proximity to the injector well did not affect the oil saturations above the injection interval (~1510 ft) 19 , where two of the more permeable samples originate (1450.00 and 1484.95). By comparing data from 10W-9 (drilled in 1994) and OB-7 (drilled in 1999), a 5% reduction in oil saturation was observed between 1540 and 1750 ft, whereas a 4 to 6% increase in oil saturation was observed in diatomite intervals below 1790 ft 19 . Wilt and others 19 were especially interested in intervals containing high percentages of opal-A (i.e, not sandy diatomites) because of their higher oil saturations as indicated by their higher resistivities.…”

“…By comparing data from 10W-9 (drilled in 1994) and OB-7 (drilled in 1999), a 5% reduction in oil saturation was observed between 1540 and 1750 ft, whereas a 4 to 6% increase in oil saturation was observed in diatomite intervals below 1790 ft 19 . Wilt and others 19 were especially interested in intervals containing high percentages of opal-A (i.e, not sandy diatomites) because of their higher oil saturations as indicated by their higher resistivities. Their observations correspond to the distribution of fluids as determined in this study.…”

“…This relationship indicates that the oil saturation decreases as the proportion of permeable pore types increases. The study well, OB-7, is only 20 ft from a water injection well, 10W-9, that has been in operation since 1994 19 ; the permeable pore types were likely swept by injection water. Crosswell electromagnetic survey results indicate that the proximity to the injector well did not affect the oil saturations above the injection interval (~1510 ft) 19 , where two of the more permeable samples originate (1450.00 and 1484.95).…”

“…The study well, OB-7, is only 20 ft from a water injection well, 10W-9, that has been in operation since 1994 19 ; the permeable pore types were likely swept by injection water. Crosswell electromagnetic survey results indicate that the proximity to the injector well did not affect the oil saturations above the injection interval (~1510 ft) 19 , where two of the more permeable samples originate (1450.00 and 1484.95). By comparing data from 10W-9 (drilled in 1994) and OB-7 (drilled in 1999), a 5% reduction in oil saturation was observed between 1540 and 1750 ft, whereas a 4 to 6% increase in oil saturation was observed in diatomite intervals below 1790 ft 19 .…”