New design mud recycling system is the effective solution for controlling solid phase in Oil based Mud (OBM) and the level of ecological danger of drill cuttings: combination of centrifugal drying system for drill cuttings and removal of the colloidal phase by a chemical-mechanical method using a high-performance decanter centrifuge. This solution was implemented as a part of a campaign to drill horizontal wells of lightweight construction at the Prirazlomnoye field of OOO RN-Yuganskneftegaz.
The vector of the oil and gas industry development shows an intensive increase of investments into well construction in difficult geological conditions, with a significant coefficient of complexity. In this regard, special attention is paid to all the newest drilling technologies, allowing to ensure trouble-free drilling. Drilling fluids are also not an exception, allowing minimizing the risks of such complications as unsatisfactory hole cleaning, stability of clay rocks, high friction coefficient, fracturing in permeable rocks and others. One of the optimal solutions for the design of complex wells is the use of oil based muds (OBM).
The use of mud recycling equipment has significant results:
- Significant decrease of low gravity solids content;
- Reduction of wastes to the pits;
- Solids control efficiency increase above 90%;
- Significant OBM cost reduction (more than 2 times);
- Stable mud properties while drilling when it is reused several times.
In current drilling practice, there are special requirements to OBM imposed by essence of the construction process of complex wells. These includes the physical and chemical properties and environmental aspects of the hydrocarbon base, the capability of controlling the rheological parameters of the OBM systems at high pressures, temperatures, and a large amount of a colloidal solid phase. This is especially true when using the latest technologies of directional drilling, bits, logging, which allow to significantly increase the mechanical and commercial penetration rates. But it becomes obvious that the technical and technological requirements to equipment and technologies of solids control also need to be changed in accordance with the level of other drilling technologies.
The current solids control technologies, in most cases, are not oriented to the specific properties of OBM, which is a result of the universal approach to the equipment design of for mass drilling, aimed exclusively at traditional solutions. Nevertheless, in the conditions of the intensively developing OBM market, caused by general tendencies in the complication of drilling, the technology and equipment of OBM cleaning from the solid phase has already undergone a number of positive innovative changes. A vivid example here is mud recycling technology and equipment, which combines solutions for drying of drill cuttings coming from the first stage of solids control equipment and mechanical cleaning of the OBM from the colloidal solid phase by flocculation and centrifugation. As a result, it is possible to resolve two tasks at once: to increase the efficiency of the solids control system and significantly shift the hazard class of drilling wastes to non-hazardous (class IV hazard of solid drilling waste with a hydrocarbon phase content of less than 10%). For the first time in Russia, the application of this technology occurred in 2014 in the entity of Verkhnechonskneftegaz PJSC Rosneft in Eastern Siberia while drilling the interval of the production well of a horizontal well. Further mass introduction of the technology occurred in 2016-2017 at the Prirazlomnoye field of OOO Yuganskneftegaz PJSC Rosneft in Western Siberia while drilling wells with an optimized 2-column design.
Efficiency of solids control system and prerequisites for the development of mud recycling Equipment
Cleanning of the drilling fluid occurs on the solids control equipment with the removal of solid phase of a certain size at each stage, sequentially as the drilling fluid moves from the well head to the tanks. The standard solids-control system includes the following pieces: a group of shale shakers, a mud-cleaner with desander and a desilter, and a decanter centrifuge. Each component is designed to extract solids of a certain size. Picture 1 shows the distribution of particle sizes and the solids-control equipment capable of their extraction. The minimum particle size that can be extracted with mechanical methods is limited to 10 μm.
At the same time, low-gravity solids remain and accumulate in the mud that can cause increase in mud rheology, inability to maintain the required concentration of bridging materials and low drill-in quality of reservoir.
Until recently, the only method of low-gravity solids control was dilution with base oils that produced a negative effect on OBM volumes and costs. The experience of production drilling with application of OBM in a number of fields in the Russian Federation, in most cases confirms the insufficient effectiveness of the solids control equipment, as a result, when the OBM is being reused while drilling of a sequence of wells, there is a tendency of accumulation of the solid phase and growth of rheology. As a result, we have to look for a compromise between the acseptiable parameters of the drilling fluid and the economics of the project.
The mud recycling technology and equipment for fine solids control of OBM
The technology and equipment of mud recycling along with extraction of low-gravity solids from drilling fluids aims at the reduction of a hazard class of cuttings discharged into pits and includes 3 main stages:
- Extraction of OBM from discharged drill cuttings using centrifugial cuttings dryer. First drilled cuttings are dried in shale shakers after which the cuttings and the remaining OBM enter a high-speed vertical centrifuge where solids are separated from fluid. The solids are discharged into a pit while the effluent is collected in a tank. The hydrocarbon content in the separated cuttings is reduced from 50% to 5-10%. However, the solids content in the effluent is still rather high so it has to undergo further treatment.
- OBM treatment with the flocculant and preparation for centrifugation. After the flocculant is added to the invert-emulsion drilling fluid and thoroughly mixed at a set temperature, micelles start to interact with solid particles. In the final part of this chemical process, the solid particles interact with polymer chains forming large solids conglomerates.
- Extraction of colloidal solids using a high-speed centrifuge. The centrifuge provides separation to a solid component that is to be discharged into a pit (hydrocarbon content reduced to 15%) and a cleaned fluid with low solids content that goes to a circulation system.
To reduce the LGS amount in the mud, the fluid from the circulation system goes through the same process bypassing the cuttings drying stage. No chemical treatment is needed except for the cleaned fluid weighting up to the required density. The invert-emulsion fluid retains all its properties. After the treatment cycle is complete, the cuttings and extracted solids are classified as low-hazard waste (Class 4 as per bioassay results) so that they can be disposed of in pits.
Application of the mud recycling technology at RN-Yuganskneftegaz sites
In 2016-2017 RN-Yuganskneftegaz decided to pilot test the mud recycling technology in 12 wells of optimized design at the Prirazlomnoye and Priobskoye fields. The optimized 178/140-mm production casing for horizontal wells consisted of two production casing sections and a liner with an average length of 3300m. This approach was supposed to reach a 30% reduction of well construction time. To realize the project it was necessary to implement a complex of innovative solutions including geomechanics, rotary steerable systems (RSS), LWD technologies, an oil-base mud, and solutions for OBM cleaning and drilling waste reduction.
Another important decision was to use the mud recycling technology. While oil-base muds were expected to provide a higher ROP, create optimum conditions for downhole equipment, maintain wellbore stability, and reduce drilling risks, the mud recycling system was to solve the following tasks:
- Maintain OBM properties throughout its multiple uses;
- Reduce OBM cost, as well as mixing and disposal volumes;
- Ensure compliance of waste with hazard class 4, similar to WBM waste.
At the time of writing this article 7 wells were drilled, that made it possible to analyze and draw certain conclusions with the help of the received results.
To assess the effectiveness of the technology, the following parameters were compared:
- Volume and mud on cuttings at the inlet and outlet of the cutttings dryer;
- Volume and mud on cuttings at the outlet of the high-speed centrifuge;
- Volume and parameters of OBM before and after the mud recycling process.
The analysis results were used to compare the drilling fluid properties before and after recycling and calculate a fresh fluid volume that might have been required for dilution if the mud recycling technology had not been implemented.
The first well was drilled with a freshly-mixed OBM, and the remaining mud was reused in the next well. The recycling equipment was applied to treat 20-30% of the mud from the previous well. Besides, the OBM was returned by extraction from cuttings in the cuttings dryer followed by the effluent mud recycling.
The results of the mud testing in one of the wells showed the following particle-size distribution: D50- 4μm, D90 20μm. In other words over 50% of solids were 10μm in size so they could not be extracted by standard solids-control equipment and they were building up.
The drilling was performed using the 1.30-1.35 g/cm³ mud. Starting with the third well, the mud recycling technology and equipment was used for partial cleaning of the OBM before drilling the interval. This strategy helped maintain the 17% colloidal solids content till the end of the interval, as well as reduce the dilution volume by 225 m³ (Picture 3). The dotted line in Picture 3 indicates the OBM volume that combines the volume of the mud recycled from cuttings and the volume required to dilute the solids down to 15-17% – the level reached through recycling. Thus the mud recycling system enabled cutting down the OBM volume.
Already in the third well, there was an uncontrolled growth of LGS content from 17 to 20%, that caused an increase in plastic viscosity by 20% (from 48 to 59 cP) and in thixotropy coefficient by 13% (from 2.4 to 2.7). The mud recycling system used for partial cleaning enabled mud rheology reduction and control.
Picture 3 shows the balance of volumes with a comparison of the realized approach with and without recycling at the time of the completion of the drilling of the well. The graph shows the dynamics of the change in the concentration of fine solid phase with the size of less than 10 microns (LGS) in the drilling mud at the end of drilling, which, in addition to the drilled rock, includes bridging agent calcium carbonate. Based on the results of measuring the drilling mud after cleaning with mud recycling system, the content of the fine solid phase in the treated volume was reduced to a value not exceeding 8%.
The solids control operations were accompanied by the extraction of OBM from cuttings discharged in the pit. The cuttings were then separated in the cuttings dryer after which the effluent was given extra treatment using the recycling equipment for a better extraction of solids. This approach made it possible to recycle about 233 m3 of mud or 17% of total mixed OBM during the drilling of 7 wells.
The economic evaluation of the project, based on its geological and engineering features (drilled-in section containing low-pressure-gradient permeable formations, high drilling fluid infiltration into the productive formation), shows reduction in average OBM costs for each subsequent well, reaching 46% on the seventh well (Picture 4) with a tendency of further decrease. This is due to the reuse of existing mud, as well as the extraction of mud from cuttings. The analysis of each separate well shows that the reuse of mud helped reduce the mud costs by half. The recycling technology can reduce the OBM cost by another 20%. Besides, there is a substantial reduction in volumes to be disposed. The difference is most evident through comparison with water-base muds that are almost entirely disposed of in pits after the completion of interval drilling.
The use of OBM in combination with the recycling equipment and technology made it possible to achieve the following results:
- Average cost of OBM for each well reduced by more then a half;
- Volume of the used mud reduced by 469m³(233 m³ of OBM extracted from cuttings and dilution volume reduced by 236m³);
- Effectiveness of the solids-control system increased to over 90%;
- Amount of drill waste dumped into a pit reduced by more then a half;
- Environmental load reduced due to cuttings transfer to a low-hazard waste category (hazard class 4) so that there is no need to change the disposal method;
- Stable and controlled parameters of reusable OBM.
In the near future, the current desire to increase drilling efficiency against engineering limitations of standard solutions will lead to a wider use of oil-base muds for drilling wells. For that reason there is an increasing demand for solutions that can make oil-base muds more economically attractive and overcome certain difficulties connected with controlling their parameters in the process of their reuse, as well as ecological considerations. The advanced RHE-USE recycling technology makes it possible to preserve technological effectiveness of reused oil-base muds and to ensure compliance of waste with high ecological standards.