RAVVA

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Innovating Development | Polycrystalline Diamond Compact Bits

Ravva - Innovating Development

Polycrystalline Diamond Compact Bits

The use of Polycrystalline Diamond Compact (PDC) bits in Ravva has consistently resulted in attaining higher Rates of Penetrations (ROPs) over long intervals and without significant wear as compared to Tricone bits in the same region. The PDC bit can also be re-used in a greater number of wells than the Tricone bit.

A PDC bit has no moving parts (i.e. there are no bearings) and is designed to break the rock in shear and not in compression as is done with the roller cone bits. Rock breakage by shear requires significantly less energy than in compression; hence less weight on bit can be used, resulting in lesser wear and tear on the rig and the drill string. A PDC bit employs a large number of cutting elements, each called a PDC cutter. The PDC cutter is made by bonding a layer of polycrystalline man-made diamonds to a cemented tungsten carbide substrate in a high pressure, high temperature process. The diamond layer is composed of many tiny diamonds which are bonded together at random orientation for maximum strength and wear resistance.

In general, the PDC bits provided ROPs ranging from 25 to 50 m/hr compared to the Tricone bit, which provided ROPs ranging from 5 to 30 m/hr in different hole sizes. Also, the Tricone bit has a limitation on the bearing life, leading to bearing failures requiring the unwanted pull out of hole (POOH) activity to change the bit unlike PDC bits. This also leads to higher wear and tear in Tricone bits.

The PDC design is affected by:

1. Body design: steel-bodied or tungsten carbide (matrix)

2. Cutters Geometry: Number, size and spacing of the cutters Back & Side Rake

3. Geometry of the bit in terms of number and depth of blades

4. Diamond table Substrate interface Composition & shape

PDC Bits Applications:

1. PDC bits have been typically useful for drilling long, soft to medium shale sequences which are a low abrasive. In such formations they typically exhibited high ROP and extended life, enabling entire sections to be drilled in one run.

2. PDC bits are not usually appropriate for highly abrasive well cemented sand sequences. When drilling tight siliceous formations the incidence of PDC chipping and breaking is dramatically increased resulting in less than expected ROP and bit life.

3. When drilling heterogeneous formations containing alternating shales and or shale limestone sequences, the use of PDC bits has proved to be encouraging. This bit incorporates the use of back-up diamond studs behind the PDC cutter. When drilling harder abrasive strings, the diamond stud absorbs the increased weight required to drill the stringer and prevents premature damage and wear to the PDC cutter.

Ravva - Innovating Development

4. With the use of mud motors, the use of long tapered profile bits is considered. In addition, radial

jetting bits reduce the potential for friction induced high cutter temperatures when run on a motor, thereby reducing the temperature degradation of the cutter.

Logging While Drilling Technology and Rotary Steerable System

The Logging While Drilling (LWD) technology and Rotary Steerable System (RSS) has been used in the drilling industry for some time. These techniques were introduced for the first time in India in the Ravva block. Triple combo LWD tools and RSS tools were run in the RX-10 well successfully for the first time and opened the way for their confident use in future wells. Triple combo uses three LWD tools in conjunction for acquiring three kinds of petrophysical data, i.e. resistivity, porosity and density measurements.

Logging While Drilling Technology

The LWD technology was developed primarily for use in high angle wells, as the conventional wireline logging tools would no longer fall under their own weight or ‘free fall’. Generally, in wells with inclination up to 45°, the wireline logging tools reach the bottom on their own. When the inclination exceeds 60° in an open hole (70° in cased hole), the logging tools do not reach the bottom and the ability to get logging tools becomes a function of hole conditions such as how clean the hole is and the roughness of the wellbore wall. As the number of high angle wells increased, the need for developing new logging methods was felt and as a result the LWD technology was invented. In this technology, the logging tools are an integral part of the drill string and acquire data when the drill string is actually drilling. This is completely opposite to the conventional wireline methods, where data is acquired after drilling and a separate run is required. Logging data is transmitted to the surface using mud pulse technology and a computer interprets it. Some distinct advantages of the LWD are as follows:

Logging data is available while drilling to help pinpoint target formations, allowing more accurate placement of directional wells

LWD can provide early reservoir evaluation and Delineation Casing or core point selection is made easy LWD logs can be used for pore pressure prediction LWD saves rig time as it eliminates separate logging runs

However, the technology also has its disadvantages. In areas where drilling speeds are high and the directional Measurement While Drilling (MWD) is being used to orient down hole motors, the logging data will be limited. Another limitation of LWD is that the data can only be transmitted at a certain rate. Hence, the use of LWD tools is situation specific. Sometimes their use is justified by the cost savings in terms of saved rig time but sometimes their rental costs and risk associated with their use renders them uneconomical. The LWD data can also be stored in the memory chip of logging tools and downloaded into a computer on a trip. The rationale behind their use in Ravva was to save rig time and thus, the cost of the operation. Since RX-10 was an exploration well and was drilled using Synthetic Oil Based Mud (SOBM), all this data was required to evaluate LM 100-40 reservoir, LM 10 sands and geological prospective target of the well. Hence, to acquire all this data in a single run and save time, it became necessary to run triple combo LWD in 8.5” hole. The LWD run was successful and provided good quality data to fulfil the reservoir requirements. The same LWD triple combo was run in the RX-8 exploratory well again to evaluate MM30 & M20SB sands in real time.

Ravva - Innovating Development

Observing the success and cost saving in the formation evaluation in the two previous wells (RX-10 & RX-8), it was decided to run the triple combo again in the RB-4 appraisal well. The angle was high for the well and it was difficult to run wire line after drilling, so it was an added advantage to use the triple combo LWD in the well. Rotary Steerable System (RSS) RSSs are different from other drilling systems as they allow active steering of the bit while continuously rotating the drill string. They permit the guidance of well trajectory in terms of inclination and azimuth, while rotating the drill string. As a result, the directional well can be placed within optimal reservoir position and orientation. Conventional drilling techniques include drilling with motors in which hole placement is not easy in complex reservoirs due to their inability to navigate and rotate at the same time. The figure shows different types of Bottom Hole Assembly (BHA) using conventional Positive Displacement Motors. On one hand the RSS has applications in the high-cost extended reach market and on the other; it provides a cheap system for onshore marginal properties where directional control is critical.