Water Saturation Calculation Using Neutron Log - Study Case on Well in Tanjung Field, South Kalimantan

Abstract

Water saturation (Sw) calculation based on data logging has been used since the 1940s where the basic formula is based on the resistivity log (resistivity log was run for the first time in Pechelbron field in France by Conrad and Marcel Schlumberger). The simplest formula for water saturation calculation was proposed by Archie (BSc in Mining and Electrical Engineering University of Wisconsin) in 1942 who assumes lithology of calculation on clastic, clean sandstone (does not have or nearly zero clay content). Over time, several formulas were proposed for improvement where the presence of clay content is taking into account. Furthermore the formula develops by taking into account how clay exist which can be structural, laminated, and dispersed. In principle, water saturation formulas which are developed are based on formulas proposed by Archie. Even though Archie equation is simple, implementing the water saturation calculation requires parameters input obtained in laboratory  measurements such as formation water resistivity (Rw), rock cementation value (m), saturation value (n), and rock tortuosity value (a) constant. Determination of the value of the Archie input parameter is critical for best estimation a water saturation value. In practices due to time or budget constraint, water saturation calculation sometime does not use data (Rw, m, n, a) from the laboratory measurement. Petrophysicist, has an option and tend to use Rw based on calculations or estimation but not from  aboratory, one of which by pickett plot. Pickett plot method was proposed by a mathematician named Dick Pickett in 1966 (SPE). Values of m, n, and a in many occasion uses a database value that is common, for example as m = 2, n = 2, and a = 1 for sandstone reservoir. On this writing, the authors propose a simpler method or approach compare to Archie method which uses a resistivity log to calculate water saturation values. The alternative method of this writing proposed is to calculate water saturation value using a  eutron log. In oil and gas industry, neutron log is often used as qualitative data together with a density log. These two logs are often seen as the value of the crossover where the higher the separation value the more it indicates the gas hydrocarbons, and less separation value indicates the oil hydrocarbons. If there is no separation value or even inverted crossover value of the density neutron log, it may indicates the presence of water. Starting from this basic idea, the writer tries to develop it further, so that the neutron log can be used quantitatively to calculate water saturation estimate. An examples of the water saturation calculation using log neutrons will be applied to the Tanjung field (South Kalimantan), where the average water cut of oil production has been increase up to 94% recently. The value of calculated water saturation using neutron log method will be compared to water saturation using resistivity method (Indonesian Equation). As validation for water saturation calculation, water cut production will be used for the well. By using this new method, petrophysicist is expected to calculate the value of water saturation easily without having to do or wait a laboratory analysis result of water samples and rock samples. Of course, determining water saturation value using the density log does not make the determination of water saturation using the resistivity log incorrect, but instead an alternative or as a comparison of one method with another. Also, the result of Sw calculation using neutron log does not mean to make Archie formula wrong, but for comparison or second opinion purpose.