On the Need to Determine the Specific Gravity of Oil Directly in Wells
Abstract Generated abstract
The paper argues for determining the specific gravity of oil directly in wells as a way to verify reservoir oil property analyses and reduce the labor associated with repeated sampling and laboratory work. It explains the standard calculation of reservoir specific gravity from surface specific gravity, oil volume coefficient, gas content, and gas density, then proposes using direct bottomhole measurements to check whether samples and analyses are acceptable. The author also notes that when high-quality samples cannot be obtained, direct in-well specific gravity measurements can be used to calculate the oil volume coefficient, and suggests a spring-balance type instrument as a practical measuring principle.
Full Text
On the Need to Determine the Specific Gravity of Oil Directly in Wells
P. A. Pukhtyakov
(Presented by Professor A. V. Aksarin)
At present, to verify the results of studies of the physical properties of oil under reservoir conditions, only repeated analyses are used. Here, however, it should be noted that the taking of samples for these analyses and the analyses themselves are very labor-intensive. In order to reduce labor costs in this direction, it is proposed to determine the specific gravity of oil directly in wells.
As is known, the specific gravity of oil under reservoir conditions is usually determined by the formula
\[ \gamma_{\text{pl}}=\frac{\gamma_{\text{pov}}}{b}+\frac{f_v \cdot \rho_g}{1000b}, \tag{1} \]
where \(\gamma_{\text{pov}}\) is the specific gravity of oil under surface conditions \((g/cm^3)\), \(b\) is the volume coefficient of oil, i.e., the ratio of its volume under reservoir conditions to the volume under surface conditions,
\(f_v\) is the gas content of the oil, referred to volume \((cm^3/cm^3)\),
\(\rho_g\) is the gas density \((g/liter)\). If the specific gravity of oil calculated by this formula corresponds to the specific gravity obtained by direct measurement in the wellbore, it may be asserted that the oil sample for analysis was taken properly and the analysis was performed with sufficient accuracy. In other words, the results of this analysis may be considered acceptable. If, however, discrepancies are found in the values of the specific gravities of the oil obtained by different methods, then the results of the study cannot be considered acceptable.
But this is not all. There are cases when it is impossible to take a good-quality oil sample from a well, while it is necessary to know its volume coefficient. The best way to solve this problem remains the determination of the specific gravity of oil directly in the well at its bottomhole and the calculation of the volume coefficient by the formula described above, reduced to the form
\[ b=\frac{\gamma_{\text{pov}}}{\gamma_{\text{pl}}}+\frac{f_v \cdot \rho_g}{1000\gamma_{\text{pl}}}. \tag{2} \]
In this case, the gas content per \(1\ cm^3\) of oil should be calculated on the basis of the gas content per ton of the oil being produced.
The instrument for determining the specific gravity of oil directly in a well should apparently be constructed on the principle of spring balances. Indeed, if one takes a load of constant weight and, attaching it to a spring, lowers it into various liquids, the extension of the spring will be the greater, the lower the specific gravity of the given liquid.