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Sunday, December 13, 2015

Black Oil vs Compositional Modeling

We often hear the Reservoir Engineers talking about Black-Oil model and Compositional Model. What are these? How do they compare? 

All these are different fluid model or the part of numerical simulation model that handles how the fluids, that is, oil, water, gas and any other substances would behave at different pressure and temperature.

Black Oil model is relatively simpler model with only three fluids - oil, water and gas. 

An extended version of Black-Oil model, called Extended Black Oil model takes are of volatile oil in gas and dissolved gas in oil.

A Compositional Model  on the other, tracks any number of components the fluid might actually has - like different hydrocarbons (C1, C2, C3, C4. C5, C6, C7,... etc), H2S, CO2, Water etc. These models are complex and takes lot more computing time than Black-Oil models.  

Brain Deposit !!

In the recent visit to the USA while addressing a large crowd of the Indian diaspora of the Silicon Valley, Indian Prime Minister Sri Narendra Modi mentioned an interesting term - "Brain Deposit". For him, the flow of talents from India to other countries is not "Brain Drain", rather "Brain Deposit", that he hopes that one day will return to "Mother India" with interest, i.e. with enhanced expertise. That`s what he calls "Brain Gain With Interest". According to him, that day is perhaps now or in the very near future.

Does it apply only to the IT sector or to our Oil&Gas sector as well? The Indian E&P companies including Oil India and ONGCL have experienced "Brain Drain" for quite a long time. These companies groomed talents in various fields like geosciences, reservoir engineering, production engineering, drilling etc from the very nascent stage. They hire fresh graduates, often with degrees with no direct relevance to the industry and spends time, money and man-hours to train them. After working and gaining experience for few years, many of them leave the company and the country in search of better opportunities. The time, money and man-hours spent apparently goes down the drain. The expertise and skills imparted by the Indian companies are utilized later by the foreign companies. Are these "Brain Drain"  or "Deposit" ?  

Tuesday, June 2, 2015

Volumetric Method of Reserves Estimation

The volumetric method of reservoir is a very simple and yet elegant mathematical tool that gives a fairly accurate idea about the volume of hydrocarbons in the reservoir. Its accuracy depends upon a correct estimation of certain physical properties of the reservoir viz. Porosity , Reservoir thickness .

In the simplest mathematical terms , the volume is determined as 

Where , V is the volume of the hydrocarbon in reservoir
              A is the area over which the reservoir is spread
              h is the thickness of the reservoir
              phi is the reservoir porosity
              Swc is the connate water saturation
              Bo is the formation volume factor




Sunday, December 14, 2014

Gas Cap Blowdown (GCBD)

Gas Cap Blowdown (GCBD) is a process of depressurizing the gas cap of a reservoir.

GCBD is applied to extract the gas available in the gas cap of a reservoir after full extraction of the oil reserves. After the operator is convinced that the remaining oil volume can not be commercially extracted, the pressure energy preserved in the gas cap is no longer required. Hence, it can be allowed to deplete or depressurize and in the process gas can be produced for sales or other applications.

GCBD is carried out especially in oil-rim or oil-sheet type reservoirs where oil is sandwiched between an aquifer and a gas cap. It is carried out in several stages as described below.

Stage-I: Stop pressure maintenance, that is, shut-down any gas and water injection wells, while keep producing the oil producers as normal.

Stage-II: Open-up the high GOR wells previously shut-down. Keep producing oil and gas from other existing wells. GOR will increase in all the wells and eventually they will all become gas wells.

Stage-III: The down-dip wells will be watered-out. Shut-down any watered out wells. The water-front will gradually move up the reservoir watering out the gas wells one by one. At the end, the crestal most well.

The life-cycle of an oil rim under-going GCBD is shown in the following series of figures. Abbreviations: OP=Oil Producer, GI=Gas Injector, HGOR=High Gas Oil Ratio, GOR=Gas-Oil-Ratio, GP=Gas Producer.

Figure-1: At the beginning of field life. 

Figure-2: In later stage of field life. 

Figure-3: At the end of oil producing phase. Decision time for GCBD.

Figure-4: Stage-I of GCBD started.

Figure-5: Stage-II of GCBD started.

Figure-6: Stage-III of GCBD.

Figure-7: At the end of GCBD process.

Sunday, November 30, 2014

Reserve Estimation

One of the main factors that determines the viability of an investment in an oil and gas field is the volume of the hydrocarbons present in it. In simple terms, we would like to invest to produce oil and gas, if only the money that can be generated by producing the hydrocarbon from that field generates profits. As typical investments in a field run into billions of dollars in terms of facilities and operation costs, it becomes imperative to have a good idea about the hydrocarbons present.

Initially, when we set out to develop a field, we do not have much idea about the reservoir and its characteristics viz. porosity, permeability, areal extent, thickness etc. As we continue to drill wells and develop the field, we get more and more data about the reservoir.

Correspondingly, at the beginning we have very little idea about the crude volume in place at the reservoir and so we employ some relatively simple and unsophisticated methods for determining the volume. These methods have a high degree of uncertainity associated with it. This means that there is a high probability that the volume which we have estimated from these methods may have a wide variance from the actual volume in the reservoir.  The methods employed at this stage are : Analogy and Volumetrics.

Later , as we have more data from the reservoir , we can employ more sophisticated and reliable methods for generating the volume of hydrocarbon. The results that we generate from these methods have a higher degree of certainity and we can expect the results to be closer to the actual values.The methods employed are : Decline Curve Analysis , Material Balance and Numerical Reservoir Simulation.