Evaluation of Reservoir for Production Allocation using Reservoir Fingerprinting: A Case Study
Authors: Nengkoda Ardian , Mandhari Musallam Sulaiman , Widojo Setiawan , Al-Hinai Zaher Mohammed 
Published: February 2016

The Z gas field of PDO is currently contributing of Oman's gas requirements for LNG export. The field is producing from the rich gas-condensate bearing Reservoir A and lean gas bearing Reservoir B and Reservoir C sandstone. The total number of wells until to date is 22 development wells drilled in the field, of which 11 wells are on production, 6 wells produce from Reservoir A only, 1 well from Reservoir B only, 3 are commingled Reservoir A, B and C, and 1 well from Reservoir A and C. The average formation split from the field predicted 45% Reservoir A, 20% Reservoir C and 35% Reservoir B, and in specific is vary from well to well and however it is depend on the drawdown. The primary surveillance tool is used to allocate a relative production for two reservoirs by utilizing a MPLT surveys. And recently, a campaign of multi-phase flow meters coupled with PLT's was initiated. Gases in Reservoir B and C reservoirs of Oman have very different carbon isotope character than those in Reservoir A. These differences are unique and can be used to calculate mixing proportions of commingled production. The proposed solution to this problem was the derivation of three models that accounts for variable isotope and composition characteristics, and trending in which all such data could be correlated. A lesson learnt and limitation is captured from this fingerprint job.

Introduction

Gas fingerprint by means gas chromatography and isotope analysis can bring solution to the problem of properly allocating gas production to specific reservoirs when more than one formation is commingled in a well.

The common allocation methods involving zone isolation are limited because the commingled flow rates obtained on the surface are not necessarily representative of actual individual formation contributions. The natural gas components such as methane, ethane, propane, iso-butane and n-butane can be characterized by their stable carbon (13C/12C) and hydrogen (2H/1H) isotopic compositions, while heavy isotopes 13C and 2H are the rare component. Small variations in 13C/12C can be analysed accurately with mass spectrometer. In specific, the ratio follows the equation:

Currently, the Z gas field of PDO is contributing of Oman's gas requirements for LNG export. The field is producing from the rich gas-condensate bearing Reservoir A and lean gas bearing Reservoir B and Reservoir C sandstone. The total number of wells until to date is 22 development wells drilled in the field, of which 11 wells are on production, 6 wells produce from Reservoir A only, 1 well from Reservoir B only, 3 are commingled Reservoir A, B and C, and 1 well from Reservoir A and C. The average formation split from the field predicted 45% Reservoir A, 20% Reservoir C and 35% Reservoir B, and in specific is vary from well to well and however it is depend on the drawdown.

The primary surveillance tool is used to allocate relative production for two reservoirs by utilizing a MPLT surveys. And recently, a campaign of multi-phase flow meters coupled with PLT's was initiated. Gases in Reservoir B and C reservoirs of Oman have very different carbon isotope character than those in Reservoir A.