Next Generation Waterflood Surveillance: Behind Casing Resistivity Measurement Successfully Applied in the ‘A3-A6’ Waterflood at Elk Hills Field, Kern County, California

Abstract

The project successfully used cased hole resistivity measurements to identify, isolate and access behind-flood-front oil in wells watered out as much as 10 years ago. The ‘A3-A6’ reservoir at Elk Hills has undergone peripheral waterflood for 20 years and has a Dykstra-Parsons permeability variation of ~ 0.7. Reserve development now targets bypassed oil. In the past, determination of flood front position required open hole resistivity logs. Traditional cased-hole logs have been inconclusive as flood front monitoring tools. Only resistivity measurements provide the direct measurement to discern oil and water in this reservoir. Watered out wells are mechanically prepped and then logged with a cased hole resistivity tool. Separation between the original and present resistivity curves indicates swept sands. When the open hole and cased hole resistivity curves overlay in sands, it gives a positive indication of unprocessed, bypassed oil reserves. From this work, the ‘A3’ sand has been identified as unprocessed by the waterflood in several wells and processed in another well. Isolation and reperforation has resulted in commercial production in three wells to date. Behind casing resistivity measurement is the only logging method to successfully discern oil and water in the ‘A3-A6’ waterflood. Every wet well is now reviewed as a candidate for this surveillance technology. Results demonstrate that behind-casing resistivity measurement is a next generation waterflood surveillance tool. Introduction The ‘A3-A6’ reservoir is located on the Northwest Stevens structure in the Elk Hills field near Bakersfield, California (Fig. 1). The ‘A3-A6’ reservoir has been undergoing peripheral waterflooding since 1983.The reservoir is a series of turbidite sands with more than 200 ft of vertical thickness. These sands are grouped by increasing depth with the following nomenclature: ‘A3’, ‘A4’, ‘A5’, and ‘A6’ sands. Figure 2 is structure contoured on one of the ‘A6’ sands showing over 800 ft of relief. This structure map shows the position of the injectors and the 90% water cut line as of January 2002. The map highlights the position of three wells behind the flood front in which the new through-casing resistivity logging tool was run to identify bypassed oil. Success in the use of cased hole resistivity measurements to find and isolate behind-flood-front oil in a maturing waterflood is demonstrated. Reservoir Description The ‘A3-A6’ reservoir is located on the western limb of the Northwest Stevens anticline in the Elk Hills field. The ‘A3-A6’ sands are a series of turbidite channel sands of Miocene age pinching out toward the east. There are four major productive units ranging in depth from approximately −7,600 ft SS to −8,550 ft SS, covering an area of approximately 1,300 acres. The reservoir dips 30°-40° along the flanks of the anticline and broadens and flattens slightly moving westward down the anticlinal plunge.

Hydrocarbon Potential Monitoring in Gas Sandstone Reservoir Using CHFR and TDT Techniques

“…Because This depletion Indicator (DI) can be plotted versus depth for formation of interest, taking the value 1 as a base line, below which the formation can be considered as it is depleted: RCHFR less than ROH [10][11][12][13][14][15][16][17].…”

“…In other instances the electrodes may make contact with the casing by scratching away the coating, meaning that the CHFR measurement would be unaffected but that corrosion may be induced. Dual casing strings: CHFR is not designed to operate in dual casing strings; the number of variables involved is too complex to enable the formation resistivity to be quantified [17][18][19][20][21].…”

Hydrocarbon Potential Monitoring of By-Passed Pay Zones Using TDT and CHFR Logs

“…[1,3] In general case of shaly porous formation containing water and hydrocarbons, water saturation , Sw from TDT is : This depletion Indicator (DI) can be plotted versus depth against formation of interest, taking the value 1 as a base line, below which the formation can be considered as it is depleted: R CHFR less than R OH [10][11][12][13][14][15][16][17]. This technique is based on the fact that capture crosssection of a material is the summation of the crosssection of all its elements multiplied by their respective volume proportions.…”

Cased-Hole Formation Resistivity (CHFR) Technique Improves Hydrocarbon Saturation Monitoring in Developed Reservoirs

“…to operate in dual casing strings; the number of variables involved is too complex to enable the formation resistivity to be quantified [17][18][19][20][21].…”

“…Archie equation will be used in addition to open-hole porosity logs. The Archie equation is used to determine the water saturation (Sw) from true formation resistivity logs for clean formation, as follows: Sw (CHFR) = ( FR w / R CHFR ) 0.5 Depletion-Indicator, DI is the ratio of the reference openhole to new cased-hole Archie water saturation values DI = ( R t / R CHFR ) 0.5This depletion Indicator (DI) can be plotted versus depth against formation of interest, taking the value 1 as a base line, below which the formation can be considered as it is depleted: R CHFR less than R OH[10][11][12][13][14][15][16][17].…”