SPE/IADC-173105-MS

Drilling Mud Cooler Opens Up New Automated Drilling Markets in Hot HoleApplications

K. El Dorry, A. Coit, C. Garza Gutierrez, J. Woolums, and D. Herrington, National Oilwell Varco

Copyright 2015, SPE/IADC Drilling Conference and Exhibition

This paper was prepared for presentation at the SPE/IADC Drilling Conference and Exhibition held in London, United Kingdom, 17–19 March 2015.

This paper was selected for presentation by an SPE/IADC program committee following review of information contained in an abstract submitted by the author(s).Contents of the paper have not been reviewed by the Society of Petroleum Engineers or the International Association of Drilling Contractors and are subject tocorrection by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers or the International Association of DrillingContractors, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of PetroleumEngineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words;illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE/IADC copyright.

Abstract

Drilling automation depends on delivery of high-speed downhole dynamics data to control surfacemachinery. The two principal surface machine parameters controlled by high-speed downhole data are topdrive rotary speed and the tripping speed of drawworks. The high-speed downhole dynamics data isdelivered to surface via complex downhole dynamics electronics packages with sensors that measurevibration, loads, temperature, and pressure. As the bottomhole temperature rises above the functionalthresholds of these downhole electronics packages, the life and performance of the downhole tools andsensors deteriorate, making it uneconomical to perform closed-loop control that depends on the high-speed data.

A breakthrough in land rig mud cooling technology now allows for much lower, safer downholetemperature gradients for the safe use of the necessary downhole dynamics tools to fully automate thedrilling process. The closed-loop mud cooler was used on a series of wells in South Texas with advanceddrilling automation tools to compare the results of drilling speed, efficiency, and downhole tool opera-tional safety with the mud cooler either activated or deactivated.

During the tests, the frequency of downhole tool failures diminished from two temperature-relatedfailures per well to zero tool failures, which in turn reduced the need for bit trips and expedited the overalldrilling rate. The operator drilled the well in three days fewer than the previous well.

The high-speed downhole dynamics measurement tool that controls the automated driller at surfaceusually has a maximum battery life of 250 hours. When the mud cooler was used, the downhole dynamicstool was able to achieve 96% of its capacity (240 out of 250 hours) for the first time in an environmentwhere downhole temperatures exceeded 250°F. The previous maximum tool life achieved was only 167hours (67% of the total battery capacity); meaning run length capability was increased by 44%. Even ifdownhole dynamics data is transmitted to the surface at a rate insufficient for automated surface machinecontrol, the slower data will still allow the driller to make better performance drilling decisions. The mudcooler allows for a reduction in temperature of up to 45°F (at surface) and 21°F at bottom hole in a singlepass for flow rates up to 550 gallons per minute during the summer time.

This paper focuses on performance data and charts for the overall operations on multiple wells drilledas part of a drilling automation case study in a hot-hole application in South Texas (Eagle Ford Shale).

A basic breakdown of the mud cooler technology will also be included, as well as installation, operation,and maintenance best practices for optimal performance.

TheoryElevated and fluctuating down hole temperatures and pressures can negatively impact critical mudproperties such as viscosity gel strength, directly resulting in challenges with hole cleaning, filter cakestability, down hole tool life, and potential stuck pipe events. These challenges increase overall operationscosts and non-productive time. Hot mud’s effects on fluid rheology can also affect the life and accuracyof in-hole measuring devices such as MWD (Monitoring While Drilling) and other types of tools thatincorporate electronics or elastomer components.

These problems are especially pronounced on land where water is not readily available as a coolingmedium and mud coolers must be designed to effectively dissipate drilling fluid heat. If we reduce andcontrol mud temperatures we reduce costs, NPT, and improve life of mud pump and top drive expend-ables, ROP, bit life, and enhance hole cleaning.

Extended elastomer service life means extended length of runs and less tool costs. Temperaturestability of drilling fluids results in longer fluid life and fewer additives because the rheological propertiesof the drilling fluid are kept more stable. Cooling the mud also provides a safer working environment forpersonnel when sampling monitoring mud; especially with oil based mud systems that retain heat.Lowering the temperature in the pit room also reduces airborne vapors.

TechnologyThis mobile, fully automated land mud cooler relies on specially designed twin-plate pack heat exchangersand proven air cooling system as well as chiller technologies to continuously cool drilling fluid in a singlepass. Due to its innovative design, efficient cooling is achieved even with elevated ambient temperaturesand without the need for an external water source.

The technology works by running air cooled / chilled water and mud though adjacent titanium platesthat are connected and sealed separately to contain the individual fluid streams i.e. active mud system. Thesystem replicates conditions offshore where cold sea water is accessible while taking advantage of theefficiency of the ambient air conditions to cool the drilling mud. The two pass configuration allows hightemperature mud to be pre-cooled and then chilled again by a second stage cooler to be chilled with ahighly efficient refrigeration unit.

Figure 1—Unit on site

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Application/Process DetailsPerformance testing was carried out in South Texas in the Eagle Ford Shale. The study includes data for5 wells stretching across 2 pads. The mud cooler was operated successfully in 8.5� hole in the curve andlateral sections of all 5 wells as the rig converted to oil based mud (OBM). Temperatures ranged from

The unit was called to the rig site to support a fully automated drilling system. The downhole electronicmeasurement tools expedite the drilling process had a maximum upper temperature limitation of 300°F.In the well preceding these tests, the measurement equipment experienced a heat related failure, requiringthe mud cooler to guarantee success in these subsequent wells. Monitoring of kW and BTU/hr. of energyspent and removed from the system was monitored to better understand the unit’s performance from anenergy removal and consumption standpoint.

Performance DataAt the end of the well, the operations team operated the unit at optimized setting and reduced circulationtime during at the end of each connection from 45 minutes to 15 minutes. The battery life also reachedalmost maximum life (90%) on the downhole tools.

Pad 1 - Well 1The first well was utilized to optimize the unit’s operational parameters. Flow rate and solids controlissues were identified and improved shaker and pump capability were added to ensure target flow ratesand solids content were achieved. With larger pumps in place and fluid feed was re-routed to pull fromthe top of the mud tanks rather than the bottom to prevent introduction of additional solids that might settleat the bottom. Despite these limitations, the system was effective in reducing the bottom hole temperatureand maintaining temperature within range of the sensitive downhole electronics. An average delta drop of46 degrees Fahrenheit across the mud cooler was seen on Well 1 of Pad 1.

Pad 1 - Well 2With optimized parameters and setup, an additional 17 hours of up time on the unit was seen over the firstwell. This increase in operating time produced a reduction in average temperature of 9 degrees. The 2hours of downtime was related to an auxillary shaker requiring maintenance. Flow rates were restrictedon this well due to maintenance issues with the shaker. An average delta drop of 36 degrees Fahrenheitacross the mud cooler was seen on Well 2 of Pad 1.

Figure 2—Two Pass Configuration

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Pad 2 - Well 1One of the goals for the second pad was to significantly increase the flow rates in order to remove moreheat from the fluid. It was decided for the third well to allow the fluid to free fall into the tank to improvethroughput. Flow rate increased from 258 GPM to 499GPM on average over the previous well. To furtheroptimize the mud cooler, the return line was also re-routed to increase the cooling effect of the mud insidethe tanks as shown in Fig. 3. Instead of returning the chilled mud downstream of the feed, it was decidedto return the mud upstream in order to cool down an entire block mud creating a heat sink inside the tank.This reduced the temperature of the mud coming into the cooler, which resulted in lower final outputtemperature relative to the increased flow rate. An average delta drop of 23 degrees Fahrenheit across themud cooler was seen on Well 1 of Pad 2.

Input temperature dropped significantly compared to the last well despite the return mud coming backfrom the well was at relatively the same temperature This confirmed that higher flow rates achieved thepurpose of removing more heat. The unit averaged approximately 8% more energy removal from the mudas the flow rate increased for the well due to the use of a larger feed pump and shaker modifications.Increasing the flow rates allowed the rig to reduce off-bottom circulation time from 30-45 minutes at theend of each stand on the first pad to roughly 15-20 minutes. This improvement carried across all threewells on the second pad

Pad 2 - Well 2At the end of this well, it was noticed that the solids coming over the shaker were significantly reducedso the shaker was bypassed at the end of the well and flow rates improved significantly. This allowed forthe removal of several unnecessary connections, which assisted in improving the flow rates significantlyfor the last well. It was also decided to use 3 mm strainers instead of the original 2 mm strainers on thelast well (well 1). The downtime was due to the generator going down and a new generator was deliveredto continue operation.

Pad 2 - Well 3To calculate and analyze the value added and performance impact of the mud cooler, the unit was shutdown for a 12 hours during the operation. The data was then compared to the 12 hours after the unit wasturned back on to compare the difference in heat rise downhole. Fig.4 shows the sequence and depth wherethe unit was turned off and on. Fig. provides a comparison of all five wells. As Fig. 4 shows, slope 1 shows

Figure 3—Optimal Configuration

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a significant rise in temperature with mud cooler off compared to slope 2 when the unit was turned backon. The heat rise was .0114 °C/ft. drilled (11.4°C per 1000 ft.) when the unit was off versus .0081 °C/ft.drilled (8.1°C per 1000 ft.).

Fig. 6 shows the level of energy removed and KW consumed by the unit. The average kW consumedwas 200 kW with a maximum of 299 kW. The average BTU/hr. removed was 5 million BTU/hr.

Figure 4—Temperature Increase Slopes

Figure 5—Five well data comparison

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ResultsThe mud cooler helped reduce the drilling time by reducing the mud tank temperatures rapidly enough tocool the drill string and ultimately the formation while maintaining a steady state temperature point downhole in the drill string. During the tests, the frequency of downhole tool failures was reduced from twotemperature- related failures per well during the well previous to the mud cooler’s addition., to zero toolfailures across the five wells drilled in this performance test. This directly reduced the need for bit tripsand improved the cumulative rate of penetration. The optimized system allowed the operator to drill thewell in three days less than the well previous to this test.

The high-speed downhole dynamics measurement tool that controls the automated driller at surface hasa maximum battery life of 250 hours. For applications where downhole temperatures exceeded 250degrees, the previous tool life achieved was only 167 hours (67% of the total battery capacity). When themud cooler was used, the downhole dynamics tool was able to achieve 96% of its capacity (240 out of250 hours). The net effect on downhole run life was increased by 44%.

Overall the mud cooler had 3 main benefits to the drilling operation as shown in Fig.:By slowing down the rate at which the temperature increases downhole, the time it takes for the well

to reach critical circulating temperature (300°F) is delayed till a deeper point in the well. This is equatedto 17.5 hours of drilling time and 1000 additional feet of drilling.

Secondly, once the bottom hole temperature reaches the critical circulation temperature (300°F), it tookless time to lower the temperature of the well by circulating. This allowed the driller restart drilling soonerand reduced non-productive rig time. The cumulative average time saved per well is calculated at 51hours.

The third benefit is that the overall bottom hole temperature is estimated to be 20°F lower thanprojected temperatures at the circulation point depth due to the effects of the mud cooler. This equates tolower thermal degradation of bit cutters, the ability to run downhole tools at higher differential pressures,longer downhole tool life, all of which allows for delivering more energy to the formation and increasingROP. Fig. 7 summarizes and compares relative performance.

Figure 6—Energy removed vs. consumed

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ConclusionsBy including this technology as part of a comprehensive automated drilling solution the environmentalvariables of drilling operations can be more successfully controlled. The ability to aggressively manipulatedrilling fluid temperatures produces improved drilling conditions and supports superior performance ofadjacent technologies being employed downhole. By proving its ability to limit the negative effects ofhigh temperature fluid, the mud cooler has been confirmed as a viable solution for mitigating time andmaterial costs as well as enhancing cumulative ROP.

Figure 7—Value Summary

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