ABSTRACT
Gas condensate reservoirs are single phase at the initial condition.
It changes to multiphase when the reservoir conditions are located
between the critical point and the cricondentherm of the reservoir in
the phase envelope. This makes it unique to providing a reliable source
of energy for human usage.
The objective of this study is to use a 3D compositional model of the
Niger Delta field to evaluate the production of a gas condensate
reservoir.
This study is integrated into two main segments; the reservoir and
fluid pressure, volume and temperature (PVT) model followed by reservoir
simulation studies. The simulation involves running several development
scenarios using the 3D compositional model of the Niger Delta field to
optimize the recovery from the reservoir of interest.
Two vertical and horizontal wells were drilled in the model to study
the production of the gas condensate reservoir using two development
methods; natural depletion and gas cycling to maintain the reservoir
pressure. Production of the gas condensate reservoir by maintaining a
higher reservoir pressure through gas cycling maximized the hydrocarbon
recovery.
Economic analyses carried out on the net present value (NPV) of the
various production methods studied in this work, revealed that gas
cycling with vertical wells proved to be economical as a result of
higher cash flow and the effect of injection cost on the project
profitability.
The results of this study gave further insights into the need to
conduct detailed fluid PVT characterization and the importance of
evaluating various reservoir optimization techniques in order to
maximize recovery of oil from gas condensate reservoirs.
Keywords: Gas condensate, reservoir modeling, reservoir development methods, NPV analysis,compositional fluid characterization
CHAPTER 1
1.0 INTRODUCTION
1.1 Problem Definition
The global demand for fossil energy is increasing immensely because
of high population and development. With the unbundling of the power
sector in Nigeria, and the fact that the country currently suffers from
energy poverty complicated by low oil price or oil price volatility, oil
companies are looking for ways to minimize costs and increase
productivity.
Nigeria has an estimated 180 trillion standard cubic feet of natural
gas proven reserves, making it ninth in the world, and the largest in
Africa (World Factbook, 2014) with about a 50/50 distribution ratio
between Non-Associated Gas (NAG) and Associated Gas (AG). The Natural
gas available in Nigeria could either be associated or non-associated.
Associated gas refers to the natural gas found in association with oil
within a reservoir, while reservoirs that contain only natural gas and
no oil, have gas termed as non-associated gas.
Gas-condensate reservoirs represent a vital source of hydrocarbon
reserves which have been recognized as a reservoir type, possessing the
most complex flow and thermodynamic behaviors. Gas-condensate reservoirs
are distinguished by producing both gas and condensate liquid at the
surface. Retrograde condensate reservoirs produce gas to liquid ratios
of about 3-150 MCF/STB (McCain, 1990), or condensate surface yields
which range from 7 to 333 STB/MMCF. The condensate produced adds
economic value in addition to gas produced, therefore making the
condensate recovery a key consideration in the development of
gas-condensate reservoirs. At greater depths during exploration, gas
condensate reservoirs are encountered more often than other natural
resources at higher pressure and temperature. This high pressure and
temperature lead to a higher degree of degradation of complex organic
molecules. As a result, the more the organic materials are buried
deeper, the higher the tendency of the organic material to be converted
to gas or gas condensate. These reservoirs have grown significantly as
deeper depths are drilled to hit its targets, subsequently encountering
very high temperatures and pressures which are necessary for their
presence (Okporiri and Idigbe, 2014).
The typical pressure of a gas condensate reservoir is mostly above or
close to critical pressure when discovered. Initially, the reservoir
pressure is at a point that is above the dew-point curve, so the
reservoir is in gaseous state and only at this time does single-phase
gas exist . However as the production progresses, there is isothermal
pressure decline and a liquid hydrocarbon phase where condensate is
formed viz. when the bottom hole pressure in an existing flowing well
falls below the dew-point of the reservoir fluid. The condensate
accumulation tends to build up a liquid phase around the wellbore. It
leads to a decrease in the effective permeability of gas into the
wellbore. The low productivity associated with condensate buildup can be
substantial.
1.2 Aim of the Research
To optimize the development of a new gas condensate reservoir through
integrated dynamic reservoir simulation studies which will involve
running several development scenarios. Maximize the recovery of oil from
gas condensate reservoirs and ensure the provision of produced gas to
maintain pressure.
1.3 Research Objectives
In order to achieve the above set goal, the following objectives will be considered:
1. Develop a typical Niger Delta gas condensate dynamic reservoir model.
2. Determine the impact of geometric and harmonic averaged permeability distribution on the condensate recovery.
3. Determine the number of wells to be drilled to reach production objectives.
4. Assess gas injection pressure maintenance options and its impact on recovery.
5. Run the economics analysis of the project.