Integrated evaluation of wet gas reservoir : minimizing volumetric
uncertainties using dynamic analysis

Saidu, B

Abstract

There is a growing research effort to understand the most reliable approach in estimations of
oil and gas reserves, through different procedures such as the volumetric, Material Balance,
Reservoir Simulation, Decline Curve Analysis (Production performance analysis), which
depends on the understanding of the physical flow characterization of the formation, production
data, and recently dynamic nature of reserves. While most researchers were motivated by static
nature of dry gas reserves and hence, their reserve estimations consider parameters typical of
such reservoirs, formations where associated gas is found has continued to be a challenge
particularly in sandstone reservoirs.
In this study a novel approach of adopting integrated experimental and analytical techniques,
using digital core flooding system to establish dynamic properties of the reserves was used in
combination with analytical techniques including Volumetric, Decline Curve Analysis,
Reservoir simulation and Material balance.
Experimental study was conducted in phase I to determine rock properties, such as effective
porosity, permeability and distribution of pore size, generally regarded as petrophysical
properties, the core characterization measurement of the dimensions and weight were
performed using the Vernier calliper, weight measurement balance.
In Phase II PVT (Pressure, Volume and Temperature) analysis for gas composition and fluid
properties were also carried for a wet gas field case study (Ogba Essale). A sub set of the
sample was flashed from reservoir condition to atmospheric condition (758.31 mmhg and 82.4
f). The products (i.e. gas and oil) were analysed by gas chromatographic technique and then
mathematically recombined to obtain the reservoir fluid composition. Constant composition
expansion (CCE) test, Constant volume depletion (CVD) test were performed at the reservoir
temperature of 224.6 f. multi-stage separation test was performed at the specified surface
processing condition, the results were subsequently inputted for reserve evaluation into various
method
Phase III Involved Modelling and computer simulation Static geologic models in Petrel and
Reservoir simulation models in Eclipse 100 and 300 were built and utilized to estimate the
hydrocarbon volumes. Similarly, in this phase Declining Curve Analysis using Oil Field
Manger (OFM), Material Balance and Volumetric calculations was carried out.
Phase IV focused on Single Phase flow of Buff Bera using Navier-Stokes equations and
Darcy`s law to describe single-phase gas transport and free gas at the pore spaces. The models
were developed using water salinity representation of the wet gas field in the case study, to
simulate the performance of the natural gas reservoir in assessing the performance of
production from natural gas reservoir.
Phase V: Core flooding for two-phase liquid movements under unsteady state or steady state
circumstances and single-phase gas steady-state experiments, was conducted.
Phase VI involved the application of COMSOL-Physics, constitute the creation of a pore-scale
finite element mesh of sandstone core samples from SEM images and based on the numerical
simulation of sandstone at a pore-scale level based on experimental results
iii
Phase 7: Results analysis and discussions: The findings indicated from the characterization
(phase I) indicated for porosities of the respective core samples: Buff bera 24.55% and 20-22%,
Castle gate 29.31% and 27-29%, Boise 30.35% and 28%, Bandera Grey 19.67 and 19-21%,
and Grey Beira 20.18% and 18-21% for experimental and factory values respectively. While
permeabilities values indicated Buff Beira 458.1mD and 350-600mD, Castle gate 1434.8mD
and 1300-1500mD, and Boise 2196.4mD and 2000-4000mD for experimental and factory
values respectively, the porosity and permeability values by the experiment deviated slightly
from the factory porosity values. The experimental result showed good agreement with the
literature data under dynamic conditions, subsequent data of the Buff Berea experiment result
was implemented into COMSOL multi physics software to characterize gas transport of singlephase flow at pore scale level. Also, for this study, the Buff Bera values of porosity and
permeability were imputed for all the reservoir evaluation technique except for Reservoir
simulation of which porosity was estimated from the bulk density and sonic logs using average
grain density of 2.65g/cc, 1.00g/cc and 0.85g/cc for fluid density, 53msec/ft. for average grain
velocity and 189msec/ft. For pore fluid velocity, the net sand of the reservoirs was estimated
by applying Petro-physical cut-off (vsh=0.52, porosity=0.12). The results from the aquifer
salinity confirms that the higher the salinity of the aquifer the higher the natural gas production
and the lower the produced water as seen in the gwr vs time graph. there was a production
increase of about 50% when 0 wt% salt encroached the reservoir compared to when 10 wt%
Nacl. With this leading finding, a better characterisation of the natural gas reservoir will be
carried out for adequate evaluation of the performance of the reservoirs in the phase II of the
study.
Consequently, 75.9132 MMSTB of oil and 2,188.54 BCF of gas was obtained from reservoir
simulation, do nothing case: an additional recovery for the field is about 30.23MMSTB and
27.8BSCF of oil and gas respectively. case 1: an additional recovery for the field was about
37.21MMSTB and 26.0BSCF of oil and gas respectively. STOOIP of 1548.297365 MMSTB
and GIIP of 3007862.483 MMSCF from volumetric, EUR of 52261BSCF gas and EUR of
452.6MMSTB from decline curve analysis, and GIIP of 370.47MMSCF and STOOIP of
377.26MMSTB from material balance. the volume of initial hydrocarbon obtained from
material balance analysis and static model volume estimates are comparable and within 2 -6%
difference. The declining curve analysis and production performance analysis were carried out
and compared with a slight variation of the end volumes.
This study has utilised dynamic reservoir data integrated with various models, which can be
valuable in improving reserve estimation using multiple models compared to single models
adopted by many research and industry practices.

Citation

uncertainties using dynamic analysis. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Dec 18, 2019
• Publicly Available Date: Dec 18, 2019
• Additional Information: Funders : PTDF; Projects : Integrated Evaluation of Wet Gas Reservoir: Minimizing Volumetric Uncertainties Using Dynamic Analysis
• Award Date: Jan 1, 2019