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GENERAL GEOLOGY OF MUMBAI OFFSHORE BASINS

Introduction Mumbai basin is developed on divergent passive continental margin. Three-structure units of carbonate dominant stratigraphy with three contiguous major depressions with clastic domain. These units are following from Mumbai cost to toward Arabian Sea: 1. Shelfal horst-graben structure 2. Kon-comoria depression 3. kon comaria ridge 4. Lakshmi Laccadive depression 5. lakshmi Laccadive ridge 6. Arabian sea

General Lithostratigraphy sequence of Bombay offshore basin: Thickness Formation (meter) 1700 377 500 370 Chinchini Tapti (S1 silt sandstone) Mahim (Bombay high)** Daman Miocene Miocene Miocene Oligocene Age

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314 300 618

Mahuva (Heera, Mukta) Pipavav (Diu, Bassein) Jafrabad (Panna, Devgarh) Deccan trap

Oligocene Eocene Paleocene Cretaceous

Metamorphosed basement rocks Precambrian

Between the formation of Metamorphosed basement -Deccan trap, Deccan trape Jafrabad, Jafrabad- Pipavav, Pipavav Mahuva and Mahuva- Mahim is a hiatus present.

Western map:

offshore

basin

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STRATIGRAPHY OF BOMBAY HIGH Mumbai high is a part of Mahuva formation of Miocene age. Which consist of many few meter thickness limestone formations separating from each other by thin layers of shale formations. MH field discovered in Feb1974 is located at 160 Km WNW west of Mumbai city in the Arabian sea on the continental shelf of western India. This is the biggest & most prolific extends over an area of about 1500 Km2. It is NNW-SSE trending doubly plunging anticline with a faulted eastern limb having a throw of about 100 Meter and a gently dipping western margin.

Mumbai high is divided into two blocks. 1. Mumbai High North (MHN) 2. Mumbai High South (MHS) The low permeability zone divides the Mumbai high north and south. There is a fault between north and south, which is almost15meter wide. The estimated initial oil-in-place (OIIP) in Mumbai high North and south is 1659 MMt. The L-III limestone is the major reservoir in the field, which contains 94% of the proved reserves. Other hydrocarbon producing reservoirs are L-I, L-II and S1. Oil and gas accumulation is found in Basal Clastics and fractured Basement also.

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The field was put on production in May 1976. Since then it has undergone several rounds of development during the last 30 years. The field reached a production level of about 400,000 bopd during 1984-85 and was maintained for about 6 years. The field experienced decline in oil production from 1990 due to increase in field GOR and water cut. A number of steps were taken to arrest the decline such as gas and water shut-off jobs, side-tracking of poor producers, enhancement of water injection support, gas-lift and infill drilling. All these inputs helped to arrest the decline to some extent for some time. In order to improve oil recovery from the field a major redevelopment program was launched during 2000-2001.

Structural map of Mumbai high

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General succession of Mumbai High formation

Figure: Mumbai high geology.7

Source rock Panna formation of Paleocene- Early Eocene is main source rock of crude oil formation. Panna formation spread over the entire Mumbai offshore basin. Beside of Panna formation coral reefs are the source rocks as well as reservoirs. Panna formation Total Organic Carbons (TOC): 0.5- 20.4 % Thermal gradient is 3 degree per 100 feet. Reservoir Rock Limestone is mainly reservoir. Limestone is basically Micrite, Biomicrite and chalky type. Primary and secondary porosity are mainly. Average Porosity range: 15- 35% Average Permeability: 10 MD to 1 Darcy Cap rocks Shale is mainly cap rock in Mumbai high. But somewhere exceptionally carbonates are also as a cap rocks wherever tight limestone is present. Example - Bassein Reservoir Trap Anticlinal reversal & fault closures are mainly trap. Structure with in anticlinal reverse is mostly developed by drape sandstone. Trend of fault closures are ENE-WSW.

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MATERIAL PLANNING & DRILLING COORDINATION

Introduction Material planning and drilling coordination is one of the most important part of oil companies. Beneath the surface temperature and pressure increment is very common phenomena. However pressure and temperature gradient estimation is possible. But

during the drilling through various types of formation and reaching into the oil reservoir is very difficult task. So a drilling engineer face lot of problems while drilling related to formation nature and

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drilling instrument. But for resolving these problems chemical engineer (or Mud logger) associate with the drilling engineer. Main problem while drilling is lithological changes of formation and heat generation due to bit rotation. Thus to make ease in the drilling, we use various mud for prevention of formations damage and drilling bit. The chemical engineers do the selection of the muds chemicals for the purpose. But there should be good coordination between the drillers and chemical engineers. Mud is a mixture of chemicals, which absorbs the heat, generated by bit and maintain the pressure over the bit. Pressure maintenance is very much required on the bit otherwise high pressure of formation (oil reservoir) can push out the drilling string from the formation. The result of push out due to high pressure is known as blow out. There is another factor too, which control the drilling, as we know, heat generate due to rotation of bit and chances damage increase. For drilling optimization mud is in used. Need of things during drilling 1. Rig setup 2. Different bits size and type 3. Mud (combination of various chemicals) Function control the drilling 1. Pressure and temperature10

2. Lithology of formation 3. Heterogeneity of the formation 4. Mud used in drilling Drilling and casing policy in Mumbai High field First we do the pile hammer casing, in this casing we lower 30 casing pipe, which is called conductor pipe. It penetrates with in the loose sedimentary formation by hammering up to the refusal point. This casing depth in case of Mumbai High is around 100150 meter. Now we select the bit of 26 and drill the formation and then lower the casing of 20. This casing is known as Surface casing. It is done upto the depth of 300 meters. Further we use drill bit have 171/2 and drilled upto 1100 meter then lower 133/8casing. Next bit in use is 121/4 and drilled upto depth of 1900 meter then lower 95/8 casing. Next we use liner casing of 7 into the drilled hole of 81/2. Liner is used to penetrate into the pay zone; it is overlapped with the last casing approximately 100 meters, which is lowered 1900 meters to 2100 meters. In case Mumbai High hydrocarbon bearing strata occur at depth 1350 1400 meter. Use of liner is economically less costly and convenient in drilling. Finally tubing is done. The size of tube is depending upon the reservoir condition. If the reservoir is in good condition subsequently we can use 3 diameter tube. Other wise use less than11

3 diameter tube. Between all the casings is poured the cementing material, which stabilizes the well bore as well as prevents the formation from damage. The vertical drilling distance from the rig to pay zone is called the True Vertical Depth (TVD). But if the direction drilling is done the distance measure from the rig to the terminus is called Measured Depth (MD). The Horizontal distance between the TVD and MD is called Drift. Drift can be measured by using simple trigonometric formula or by Pythagoras theorem. ONGC deal with Measurement While Drilling (MWD) by123/4 drill bit. At 123/4 phase oil bas mud is in use because if we encounter the shale section, water base mud drilling can strike while the oil base mud passes into the section easily. But in Mumbai almost all horizontal wells are drilled by LWD.

Use of cementation 1. Prevent the dissolution of different zone; as water zone, hydrocarbon zone. 2. Hold the casing 3. Resume the next phase of drilling. 4. To safe the casing from collapse 5. Safe the well from cementation12

6. Hold the casing pipe and well head

Mud loss during the drilling Mud loss during the drilling cause by several reasons as followings 1. Pressure of formation is less then the mud weight pressure 2. Cracks presentation with in the formation (these cracks may result of high weigh pressure exertion on the formation) 3. Porosity and permeability

Well completion tools: In a typical production well the following instrument is installed down the hole. X-Mass Tree: production and gas injection operation Tubing Hanger: Holding the tubing at the surface. Tubing: Producing the crude from the reservoir. Expansion Joint: To overcome the buckling problem due to heating Flow Coupling: To maintain the laminar flow of the crude within the tubing. Sliding Sleeve: To displace the mud and increase the drawdown Packer: To isolate the annular space between production casing and tubing.13

Landing Nipple: To plug the tubing POP: To inflate the packers.

Material Planning Material means the solution used to make the drilling successful without the reservoir and formation damage. For this job a chemical engineer have to understand the problem and provide the solution. Here we do the mud (solution =solvent+ soluble) selection by considering all the function of formations and drilling. Type of solution 1. True solution (water) 2. Colloidal solution (motion of particles are Brownian) 3. Mixture Solution There is one special category of solution, which lies between the true solution and colloidal solution. It is called Tixoslove. It is a time dependent flow. If the tixosolve is in rest condition, internal structure of fluid build up strength and it behave similar to gel. But when the constant force or shear rate applied, it break down the gel structure. And it turns into the less viscous liquid. So it follow the Bingham Plastic and power low equation or n, k model. So it has property to interchange from

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Less viscouse Fluid Flow 1. Linear flow 2. Turbulent flow Fluid type

Gel

1. Newtonian fluid (Follow the linear trend between shear stress and shear rate curve) 2. Non Newtonian flow or Bingham flow Use of drilling mud 1. Coolant and lubricant and flushing medium 2. Maintain the pressure over the bit and absorb the heat produced during the drilling. 3. Provide the stability of the well bore prevent the formations to collapse 4. Make an impervious layer against the formation, which prevent the seepage of fluid. Increase or Decrease weight of drilling fluid Increase weight: it control by 1. Adding weighting material e.g. barite, ironoxide 2. Add soluble material of higher specific gravity e.g. Sodium chlorite Decrease weight 1. Remove drilled solid mechanically. It can be done by

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a) High speed shale shaker and b) Mud cleaner. 2. Remove solids chemically. So use selective flocculants. 3. Add lower density fluids

Type of drilling mud 1. Fresh water: dispersed system, PH = 7 9.5. This mud contains: bentonite, phosphate, lignosulfonate and organ

colloidal mud. But fresh water drilling mud is very harmful, in case, if the formation is contain clay minerals like montomorillonite, illite ant Bentonite. Because these clays minerals have tendency to swell and increase in volume, when exposed into the fresh water. 2. Inhabited mud: dispersed systems water base drilling mud that represents hydration of clays. This mud contains: lime mud, gypsum mud and seawater. 3. Low solid mud: non disperse system. It contain organic polymers. Benefits of low solids mud are followings- Faster penetration rate without sacrificing well bore stability - Here mud utilize Bentonite with polymers to achieve desirable flow and fluid loss properties

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- If these mud are properly formulated and maintained, no dispersant or thinners are necessary. Low solids mud provide the facilities in order to - Removing the low density solids - Improved hydraulics - Faster penetration - Easier mud maintenance and over all operation efficiency. There are many advantages of low solids mud. As - Undesirable solids can be minimized - Drilled solids fines can be chemically removed through flocculant. - Desirable spurt loss and reasonable fluid loses are attainable. - Stable at high bottom hole temperature. - Shearing thinning charactersistic produce viscosities approaching that water at the bit. - Main advantage is high viscosity and higher yield point to plastic viscosity. 4. Emulsifiers: e.g. oil& water and water& oil. 5. oil base mud: