Multidiscipline methodology for USDW determination to support well abandonments at offshore Rincon Oil Field

Abstract

To develop part of the offshore Rincon Oil Field, wells were drilled from a coastal corridor in Ventura County. Abandonment of these wells required that cement be placed across the USDW (transition from >10,000 mg/l TDS to <10,000 mg/l TDS). A blended Petrophysical / Geological approach was developed to create a USDW surface map to intersected wells for abandonment. California State Regulations and Codes (Article 3.2. Oil and Gas Drilling Regulations 2128. Drilling Regulations); stipulate “a cement plug at least 200 feet long shall be placed across the intrazone freshwater / saltwater interface, so as to isolate fluids in the strata in which they are found and to prevent them from migrating into other strata.” To determine water salinity and total dissolved solids (TDS); the traditional approach has been to utilize well logs run when the well was drilled. Commonly the Spontaneous Potential (SP) log or the Resistivity-Porosity (RP) log was used to estimate formation water salinity. In both cases, depth and log reading are needed in addition to drilling-mud properties (weight, resistivity, temperature, mud filtrate resistivity, and mud filtrate temperature), uninvaded-zone resistivity (in a wet sand) and the porosity of the wet sand are needed. Rincon wells at were drilled between 1927 and 1988. There is a significant uncertainty in the properties needed for salinity determination. Because key factors were not available in all wells, there is uncertainty in salinity calculations using either well log method independently. For the present study, data was obtained from 30 corridor wells. Fifty two SP and 54 RP readings were made from the log suites. TDS determinations for sample depths, based on standard SP and RP methodologies range from 4,000 to 40,000 mg/l with a R2 = 0.7415, but have a variance of 5000 mg/l to 10,000 mg/l around the 10,000 mg/l USDW value. To generate the USDW surface for abandonment cement placement, a four step process was employed. Step 1: linear interpolation in each well that had TDS reading above and below 10,000 mg/l TDS. Step 2: From the interpolated points, a surface was generated in a Petrel model using convergent interpolation algorithm with a 1st degree polynomial trend. Step 3: Adjust the surface where needed to ensure it did not extend below values greater than 10,000 mg/l in wells that did not have calculated values less than 10,000 mg/l. Step 4: Map the surface and display the intersection of the surface on well diagrams with stratigraphic surfaces and oil shows. Surfaces representing shallow, base, and deep cases were created. The base case map was used to locate USDW at each wellbore. The mapped surface has over 200 feet of variance in the well corridor. The USDW map was overlaid with surface topography. Stream discharge is inferred to influence freshwater distribution in the near subsurface along the shoreline. Wells of differing vintage and data quality require a more robust analysis to ensure consistent output. Mapping of statistically consistent salinity calculated from both SP and RP methods in a subsurface model. This improves understanding of USDW variance and enables development of a most-likely depth-to-USDW map that compares favorably to surface drainage patterns. When historical variations in watershed discharge are considered, there is strong support for adjusting depth of abandonment-cement placement in Rincon corridor wells.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)