№10 October 2010Table of contents Issue Archive
№ 10 (October 2010)
Amid today’s trend of searching for alternative energy sources, it is important that accepted theory is applied in stages, from its fundamental basics to precise details and clarifications. Under the current federal program to develop Russia’s scientific technological industry for 2009–2012, significant work is being done to develop and implement innovative technologies to produce hard-to-recover and unconventional resources including highly viscous oil, bitumen sands and kerogen laden rock.
By Nina Dieva, Marina Kravchenko
In this regard, scientists are attracted to the Bazhenov deposit, where the larger part of the reservoir rock consists of “immature”, non-flowing hydrocarbons. Recently, more attention has been paid to shale rock, a significant part of which also contains kerogen. The search for ways to develop such collectors is of smaller importance in the field of kerogen bearing rocks. This is because conventional methods cannot be used due to the specific nature of the reservoir rock. Yet this topic is of great interest particularly because of the fact that even in rough estimates, the quantity of hydrocarbon feedstock that may be produced from such formations under certain conditions could exceed the geological resources of movable oil.
Geologists and geochemists qualify the kerogen as part of dispersed organic matter, that is alkali-insoluble and organic solventless. The abbreviation IOM (insoluble organic matter) is used to designate this part of organic matter .
While searching for methods of treating kerogen bearing rock, it is important to take into consideration the fact that different treatments applied to these complex objects would alter the entire system’s thermodynamic state. Any rock is a complex thermodynamic system with its own component compounds, phases, state or combination of thermodynamic parameters [2-4]. The presence of kerogen embedded in this formation system adds a range of traits to it. The process of oil and gas generation from organic matter is followed by changes in the properties of the petroleum reservoir. The generation process in rock dependы on many factors, the main ones being temperature and pressure, as well as the duration of their action. Simultaneously, certain changes in temperature and pressure are mostly determined by the peculiarities of the underlying geological conditions .
This article aims to analyze the special behavior of a “fluid-filled rock with kerogen inclusions” system in the setting of the above-noted transformation process from the solid phase (kerogen) into the fluid phase (liquid hydrocarbons).
This article will describe the mathematical model of kerogen decomposition as acted upon by a chemically active agent. The first stage of the study envisions describing the changes in the porous structure of the solid part of the rock skeleton into the liquid phase by the chemical active agent which is injected with the inert liquid phase. It should be noted here that the transformation of kerogen into hydrocarbons under reservoir conditions is in fact a chain of several chemical reactions and phase transitions. But the present problem definition describes qualitation of process physics and chemistry, so the whole process is called a transition reaction and mathematically represented by quantity J (the reacting agent mass outflow during the reaction). A certain chemically active agent injected into the formation and mixing into the inactive fluid substance is a factor is provoking a dissolution reaction. We may suppose that before the reaction with an active agent, the reservoir is a matrix – a source rock with kerogen embeddings saturated with movable “mature” oil, which may be displaced from the matrix by a more movable phase – for instance, water. The presence of an active agent in displacing the fluid leads to the partial dissolution of the source rock, which is displaced from the reservoir. After chemical treatment, the reservoir matrix is a solid non-deforming coke-like skeleton. We accept that at first some quantity of liquid “mature” oil is already in the porous network.
The mathematical model describes the following process occurring at the same time:
The simultaneous filtration of reservoir fluid, injected liquid containing active chemical agent, and hydrocarbons generated due to transformation from kerogen;
chemical reaction with chemical agent consumption;
dissolution of kerogen part of rock to solid coke condition.
The multiphase flow model including chemical reaction kinetics, estimated in a qualitative way.
In the first stage of the model’s adaptation, we use the modified unstable oil and dissolved kerogen displacement model. All phases and phase components are incompressible fluids, the water phase and hydrocarbon phases are immiscible liquids and the chemical agent is water-soluble, and dissolved kerogen with “mature” oil and aqueous solution of chemical agent have the same order density.
At this stage, we do not assume that energy is released or absorbed during the chemical reaction. The process is isothermal with the formation temperature. Putting the statement this way, we merely imply a qualitative definition of kerogen transforming into liquid hydrocarbons. It is considered that an active agent reacts with kerogen independently of pressure and temperature rates. The reaction rate is specified only by the reaction parameter J. In addition to this, we shall take into consideration that the amount of dissolved kerogen is determined by reaction constant bc : fitting the chemical agent quantity J in a unit time given by the reaction factor bc. Kerogen fully dissolves until the agent has been injected into the reservoir. It is understood that entire volume kerogen has occupied fill with oil phase.
The system of equations for multiphase flow model, including chemical reaction kinetics is as follows: (1)
where m – porosity of rock;
c – active agent concentration in aqueous solution;
sw, so – water and oil saturation, respectively;
ww, wo ,w – water and oil phase rates and summary rate, respectively;
ρw, ρo, ρk – water, oil and kerogen density , respectively;
Jкн –the intensity of mass transfer per unit volume of mixture and per unit time (set with provision for full dissolution of kerogen while N cycles of formation washing with chemical agent);
bc – coefficient, defining kerogen portion reacted with active agent in reaction;
F(sw) – Bucley-Leverett function (J-function).
The numerical solution is found for system (1) with the assumption the initial time answers the initiation of chemical agent injection (25 mass percents) into nonperturbed formation (with appointed dimension, 10×1×1 meters). The value of injection rate modifies from 0.5 to 25 liters a day. Also we vary value of agent concentration in aqueous solution, from 10 to 50 percent. The rate of chemical reaction assigns with the assumption that full dissolution of kerogen occurs while 21 flushing-out cycles. The test results for accepted range of variable parameters have turn out in the following way (Fig. 1, 2).
The time required for full kerogen dissolution is about four months. The maximum value porosity is up to 44 percent. The model showed that injection volume has a weak influence on the intensity with which oil is generated from kerogen.
The chemically active agent does not react thoroughly with the rock matrix and dissolves only some of the kerogen if the intensity of the reaction is fast; an intensive injection rate results in useless rock washing without generating any additional amount of hydrocarbons from kerogen holdings in the formation. But the performance of kerogen transformation improves on the assumption the injection rate reduce.
Thus it should be said that this work, performed for the purpose of searching for innovation methods of developing kerogen formations, provides a model which is has been proved to be a tool suitable for use in the qualitative estimation of oil-field performance.
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NINA DIEVA holds a Master’s degree in Petroleum and Underground Hydromechanics, earned at Russia’s Gubkin State Oil and Gas University (RGUNG). She’s an active participant and laureate of numerous student contests held at college, city and national levels. Nina is the recipient of the Professor Isaak Charny scholarship. She’s had seven scientific reports published in the collection of papers presented at RGUNG conferences and in the Collection of Works of VNIIneft named after Academician Krylov.
MARINA KRAVCHENKO holds a Master’s degree in Physics and Mathematics, associate professor. She teaches at RGUNG at the Chair of Petroleum and Underground Hydromechanics. She also teaches at the Moscow State University at the Chair of Gas and Wave Dynamics. She authored more than 40 scientific and methodical works, including seven instruction manuals on Continuous Medium Mechanics, Underground Hydromechanics, Multiphase Flow Hydromechanics. She edited a number of book translations on petroleum engineering.