1 Introduction & Background

It is common during well drilling to encounter problems even when careful planning preceded actual site execution – drilling problems are costly. Non-homogeneous characteristic of formations is the main cause of problems occurring for a subsequent hole where earlier holes did not experience such problems. So geological conditions may differ for two wells drilled in close proximity. Caution and containment have never been the name of the game in drilling. Rather designing drilling programs based on anticipated hole problems have been the key to successful drilling operations. Overall-well-cost control for successfully reaching the target zone in drilling is dependent on planning solutions based on geologists’ understanding of the causes of anticipated drilling problems. Some of the most prevalent drilling problems include formation damage, borehole instability, hole deviation, lost circulation, mud contamination, reduced ROP (Rate of Penetration), H₂S-bearing formation and shallow gas (not mentioning equipment and staff-related problems). Selection of the right configuration of drilling rig helps solve or minimise some of the common drilling problems.

Understanding the Technique Used in Reverse Circulation Drilling Rigs (RC).

During exploration activities where special grade control is required, RC drilling rigs are recommended because of their dual wall drill rods with hollow inner tubes used for continuous and steady transfer of drill cuttings to the surface. Drill cuttings are blown through the annulus from the down-hole to the deflector box at the top of the rig. Drill cuttings then travel through a hose to the cyclone chamber, where they fall into sample bags – each bag is marked with the location and depth of the place where the sample was collected.

The technique applied in reverse circulation drilling is the simultaneous use of potential energy (though expansion on return to the surface through the centre hole) of the compressed air going into the double-walled drill pipe (between the inner and outer pipe). At the same time, it moves up, and the potential energy of the water column, both acting together and supplementing each other, drive the fluid to move. Fluid circulation becomes smooth and reliable under the influence of both these potential energies, minimising the chances of lost circulation.

Reverse circulation can be achieved through drilling fluid or improvised by using compressed air. A compressor injects air under pressure into the annular space between the inner and outer pipe of a double-walled drill pipe. Compressed air upon reaching the bottom of the drill pipe gets sprayed into the inner hole. This air injection through a mixer mechanism generates numerous bubbles in the central drill pipe, causing a reduction in density. Because of the continuous compressed air process expanding into the inner drill pipe, a column of low density gets created, which help the gas-liquid-solid (drill cuttings) mixture rise in the inner drill pipe. At the same time, the drilling fluid enters the inner drill pipe, thus creating another potential push to the drill cuttings to move from the bottom of the hole to the surface. This phenomenon occurs in the reverse circulation drilling, whereby drill cuttings get discharged through the inner drill pipe into the sandpit.

Understanding the Technique Used in Down-the-Hole Drilling Rigs (DTH).

These rigs were developed to increase the Rate of Penetration of drill in hard rocks. They may consist of some pneumatically operated jackhammer that is screwed on the bottom of the drill string, and the fast hammer action breaks the hard rock of the jackhammer. Drill cuttings and dust are removed by a fluid such as water or drilling mud. They most commonly use a pneumatic hammer that is driving a tungsten-steel drill bit for crushing hard rock. Most of these drilling rigs operate under extremely high differential pressure.

In the early days of drilling, a rig-floor rotary table turns a long string of drill pipe and drill collars to transmit rotation and thrust to the drilling bit, cutting the rock at the bottom hole. Under all conditions of depth, hole size and hard rock, the bit used to be powered this way. Innovation started when rotary power was put at the drill bit, thus eliminating the need for fuming the drill pipe and reducing surface power. This also helped minimise torsional stress on the drill pipe, minimising drill pipe wear and damaging the hole and protective casing. A down-hole motor is used to power the drilling bit. Mechanical rock destruction is achieved by using forces normal to the rock, high rotation speeds while maximising drilling rig life.

Using down-hole motors, using differential pressure available down-hole, started in 1956 and has significantly developed ever since, avoiding the costs and complexities of turbo-drills and electro-drills. The main advantage is that such a motor does not receive rotating torque from the drill pipe rotation as against conventional rotary drilling. Drilling torque is directly proportional to the pressure drop across the fluid-powered motor. The fluid-pressure gauge in the control room at the surface accurately indicates drilling-bit weight.

DTH drilling tools are the most economical and technically viable in directional drilling but are successfully used in straight-hole drilling. According to the SPE paper there have been developments that have increased the life and performance of down-hole motors. There are improvements in the performance of downhole motors, improvements in the dump-valve assembly (bypass valve assembly provided at the upper end of the down-hole motor to allow for drilling fluid flow in case motor rotation stops), the stator and rotor, and the connecting rod and bearings. All these developments of DTH tools have simplified rig operations and increased the effectiveness of directional drilling with the motors.

2 Advantages of RC Drilling Rigs for drilling optimisation

Due to injection of drilling fluid (or compressed air) through annulus and drill bit into the inner drill pipe in reverse circulation drilling technology, few general advantages are achieved:

1. Achieving Special Grade Control

There are instances in exploration activities requiring special grade control. Reverse circulation drilling rigs are the right answer to such special applications because of their dual wall drill rods with hollow inner tubes used for continuous and steady transfer of drill cuttings to the surface.

2. Achieving Desired Geological Sampling

Since the drilling fluid is directed to return up to the ground directly from the inner hole of the drill tool, the reverse circulation drilling technology provides a better ability to carry drill cuttings. This results in obtaining a clearer sample. This ability to obtain clear samples gets more pronounced when drilling in a leaking stratum. Thus, reverse circulation drilling serves the geological purpose better through more representative geology sampling.

3. Achieving Improved Drilling Efficiency At The Leaking Stratum.

When using reverse circulation drilling rigs, the drilling fluid creates a suction pressure around the bottom of the hole. This helps in the quick movement of drill cuttings at the bottom to the surface, reducing the compaction effect. This enhances the penetration rate significantly, and the drilling efficiency gets increased.

4. Reduction Or Elimination Of Drilling Fluid Leakage And Achieving Production Zone Protection.

It is well established that the annulus pressure loss is low in reverse circulation drilling. Due to this low-pressure force acting onto the stratum, the leaking of drilling fluid gets reduced or eliminated. For a similar reason, production zone protection is also achieved. As is evident, there is a large saving of quantity of drilling fluid material that would have been required with conventional drilling.

5. Reduction Of Mud Pump Operational Wastage.

When using reverse circulation drilling rigs, there is a significant reduction in the work performed by the mud pump. The mud pump is just pouring the mud into the annulus, and sometimes a small charge pump can be used. Reduction in the burden of work performed by the mud pump gets reduced greatly, and as a result, it has a prolonged life.

6. Achieving Flexible Well control.

Computer simulations and field data have confirmed that potential advantages exist for well-control procedures using reverse circulation drilling rigs. Better well-control is possible because of lower casing pressures, smaller cumulative pit gains and the ability to remove kick fluids much faster than conventional kill procedures.

Conventional well-control methods like the Driller’s Method or Wait-and Weight Method. However, both these well control methods have been developed with the basic assumptions that the well can be safely shut-in and the kick fluids can be circulated out of the well without exceeding formation fracture or casing burst limits. In reality, however, a well-control situation may occur where either of these assumptions fails to exist. This precludes the use of conventional well-control procedures. Examples of such situations are:

• When drilling below drive, formation strength is insufficient to shut-in the well if shallow gas is encountered, and the well has to flow through a diverter system (safe well shut-in not possible).
• When a very large kick occurs during drilling an intermediate hole, the well may be shut-in initially. Still, the developing high casing-shoe pressure due to gas expansion in the annulus may ultimately fail the shoe. (Bottom Hole Pressure exceeding maximum allowable pressure limit)

Under such high- and low-pressure situations, conventional well shut-in procedures would be unsafe due to the requirement of full well circulation. Under a high-pressure kick situation, the driller is forced to operate the choke to avoid pressure in excess of maximum allowable casing pressure, which, in turn, would allow additional kick fluids into the well.

7. Advantages in Primary Cementation of Wells.

Additionally, reverse circulation has been found advantageous in cementing operations. Reverse Circulation Cement Placement Technique (RCCPT) is increasingly used as an alternative technology for creating appropriate annular coverage for lost circulation zones. It is now field-proven that RCCPT can reduce circulation pressure, thereby enabling cement placement with no significant losses.

Conventional Cement Circulation Placement Technique (CCCPT) replaces RCCPT to achieve low ECD (Equivalent Circulating Density). RCCPT provides a particular advantage for cementing fractured formations like geothermal wells, low fracture gradient zones, and wells drilled for obtaining coal bed methane. It isn’t easy to carry out primary cementing for such zones using CCCPT, even if tried through lightweight or foamed cement. By deploying RCCPT, primary cementation is achieved without any adverse effect on displacement efficiency. Many published papers have reported that RCCPT has enabled cementing a weak formation above the casing shoe without allowing the plastic clays (above the weak zone) to extrude and saltwater to enter the well.

3 Advantages of DTH Drilling Rigs for drilling optimisation

Numerous DTH drilling tools are constantly being developed, each providing a significant functional advantage for new drilling challenges and adding to drilling rig efficiency. Two case studies are quoted from published SPE papers to help understand the full spectrum of advantages offered by DTH drilling rigs.

1. First Case Study under SPE Paper

According to the published SPE paper, increasing available choices for downhole tools is the single major factor leading to improved drilling performance. Over the past 20 years in the Middle East Region, the performance of Underbalanced Coiled Tubing Drilling (UBCTD) has significantly improved in terms of application selection, drilling technologies & procedures. Resultantly there has been a significant expansion in the scope of available reservoir targets.

One such technology is the 2 7/8″ downhole turbine drive motor (turbine). Historically, the choice of turbines for UBCTD had been restricted to harder formations. Still, for over a decade now, turbines have been used in these UBCTD applications in KSA – consistently producing steady drilling performance and meeting well objectives.

According to the published SPE paper, in 2018, drilling in KSA (Kingdom of Saudi Arabia) optimisation of drilling performance is carried out using a newly designed high-power 2-7/8″ downhole turbine. This new turbine is utilised in varying downhole environments, from hard and abrasive sandstone to the typically targeted carbonate reservoirs at varying down-hole temperatures. The new turbine design integrated with existing MWD (Measurement While Drilling) technology to execute the drilling operation in UBCTD with higher drilling efficiency and better economic returns. The new turbine was studied for the following parameters:

• Power delivery efficiency.
• Performance in varying reservoirs
• Turbine and MWD tool Reliability
• Directional drilling performance
• Downhole tool vibration monitoring.

According to SPE published paper, all of the above success indicators were closely monitored during 2018, and the results revealed a significant increase in overall footage drilled. BHA reliability also improved.

2. Second Case Study under SPE Paper

According to the published SPE paper, a study was carried out regarding drill string vibration rules in vertical well drilling and the idea of transformation of drill string vibration energy to drilling fluid hydraulic energy. Based on the study, a novel tool, Down-hole drill String Absorption & Hydraulic Supercharging Device (DSAHSD), is designed to further improve the rate of penetration (ROP). The DSAHSD combines the advantages of reducing the axial vibration of the drill string and generating more than 120 MPa ultra-high-pressure jet to effectively assist rock breaking. When the drilling fluid flows through this tool, part of the drilling fluid will be modulated to the ultra-high-pressure jet by periodic compression of piston-driven by drill string vibration. The process is supplemented by the ultra-high pressure jet ejected through an ultra-high pressure nozzle fixed on the bit edge in the bottom hole.

The rate of penetration gets significantly enhanced because of hydraulic auxiliary rock breaking of the ultra-high-pressure jet. This tool has been applied to more than 40 wells in four oil fields throughout China. According to the published SPE paper, the maximum density of the test drilling fluid was 1.9g/cm3; the maximum footage was 1480m, while the operation time of the tool was more than 200 hours. Resultantly, the use of DSAHSD increased ROP in the range of 11% to 831%.