Estimating Zonal Gas Contribution Using DST Gas Composition Parameters

Abstract

Abstract Individual contribution from each zone is of vital importance in wells producing from different zones from a single tubing string. Zonal production volumes are important for reserves evaluation as well as future projection with respect to processing facility. Gas analysis obtained during production phase when compared to those obtained during Drill Stem Tests can give valuable information regarding zonal contribution. Monitoring of production parameters can allow to develop a method based on thorough analysis of DST results to predict specific contribution from each zone. Bifurcation of commingled flow was based on the gradual change in key gas composition parameters. Both zones were initially tested separately and gas samples were collected and analyzed at surface. During testing, CO2 mole percentages from Reservoir 3 and Reservoir 2 were measured as 8 mol% and 5 mol% respectively. When well started commingled commercial production, initial CO2 concentration was recorded as 7.67 mol%. Factors were selected which when multiplied with initial CO2 concentrations gives the recorded CO2 concentration. The factors are the fractional gas contribution from each zone. Gradual decline in CO2 concentration suggested that the contribution from Reservoir 2 has increased over time. Results obtained by distributing the compositions received from production data according to the results obtained during well test analysis were matched by using iterative methods. Based on individual compositional contribution, volumetric contributions were attributed to each zone. Results predicted from this method were in close agreement with the results obtained from down hole logging survey performed later. Method used is general and can be applied to any well with similar characteristics. It can also be extended for three zones producing from same tubing string and reliable data from well testing is available. This can eliminate the costly requirement of down hole logging survey, production loss and risks related to well interventions. A simple yet holistic method was developed for estimating zonal contribution if reliable well test gas composition data is available. The data was compared with production data and based on the gas composition mix, zonal contribution was attributed which was also verified by down hole logging survey.