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Choosing the Right Mud When Drilling Horizontal Wells

Yevgeny Kambulov, Yuri Moisa (NPO Bureniye JSC), Alexander Penkin (Udmurtneft JSC)

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Modern status of the oil and gas producing sector in Russia is characterized by considerable degradation of the oil reserves structure. So-called difficult-to-recover reserves make up an increasing fraction of the total reserves; efficient recovery of these reserves can be ensured only provided that advanced enhanced oil recovery (EOR) technologies are applied.

In the last decade scientists were trying to develop efficient technologies for improvement of oil recovery ratio and recovery rate in the depleted oil fields. These technologies include: drilling horizontal and multi-lateral wells, side-tracking in existing wells, drilling through productive intervals under minimal overbalance conditions, application of special drilling mud compositions preserving reservoir properties.

One of the factors which negatively affect oil well productivity is inevitable contact of drilling mud with the productive formation in the process of drilling. During this contact solid phase of the drilling mud can plug bottomhole zone of the formation.

Permeable zone is blocked by the mud filtrate, which is accompanied by its physical and chemical interaction both with formation fluids and rock. Therefore, it is really important to select proper drilling mud composition which could make minimal negative impact on the formation.

Requirements to drilling mud for horizontal wells are more stringent compared to common vertical holes. It is necessary to keep the borehole stable, and at the same time to control filtration, inhibiting properties and density of the drilling mud. Drilled cuttings need to be transported along the borehole, which is ensured by rheological characteristics of the drilling mud. The present-day tendency in designing new mud compositions implies development of a water-base drilling mud having properties similar to those of oil-base mud. Major lines of water-base mud modification are related to the following factors:

  • Elimination of solid phase in the mud composition;
  • Utilization of mineral and organic additives inhibiting hydration processes;
  • Utilization of lubricating additives which reduces downhole resistance and prevents drilling tool sticking;
  • Formation of pseudo-plastic, structural-rheological properties of mud which can ensure both efficient drilled cuttings transportation and proper borehole cleaning, and excessive hydraulic friction relief;
  • Resistance to poly-mineral aggressiveness (salts of Ca2+, Mg2+) of formation water;
  • Assurance of surface-active properties of drilling mud by utilizing efficient surfactants which make drilling mud filtrate similar to the formation fluid.

Oil-base muds can prevent drilling problems by eliminating rock hydration. They have good lubricating and anti-corrosion surface-active properties. But their serious demerit is utilization of oil products making toxic impact on soil ecosystem, flora and fauna.

Use of oil products with low content of aromatic hydrocarbons as the base for drilling muds helped to considerably reduce mud toxicity, but did not solve the problem of biodegradation, especially in anaerobic environment.

Water-base drilling muds are used for drilling through productive horizons in Russia, especially in West Siberia, Tataria, Udmurtia, Bashkiria. Water drilling muds based on polysaccharide materials, biopolymer compositions in particular, are becoming more and more popular in drilling horizontal wells. NPO Bureniye JSC developed a technology for preparing biopolymer drilling mud 'Burvis' for drilling wells in depleted fields. Originally 'Burvis' mud was applied for residual oil reserves recovery in Kotovskoye field which is being developed by Udmurtneft JSC.

Oil fields in Udmurtia have been under development for more than 20 years, therefore, they have high percentage recovery (55.4%), high water cut (up to 85%), and are characterized by declining oil production.

Major oil reserves in Udmurtia are concentrated in multi-layered, extremely heterogeneous, porous cavernous fractured carbonate reservoirs; they are characterized by high viscosity due to considerable content of asphaltene-tar-paraffin compounds. Kotovskoye field was discovered in 1987. It is located in Udmurtia, on the territory of Karakulinsky region, 30 km south of the town of Sarapul.

As per the approved development plan, three productive intervals were identified in the field: Podol-Kashir-Vereian, YasnoPolyanian and Tournaisian. In 2002 eight wells were drilled: four horizontal, two directional and two vertical ones.

Two biopolymer drilling muds 'Burvis': fresh and saline ones were used for drilling through Yasnopolyanian horizon in Kotovskoye field.

The following technologies were applied in the course of operations:

  1. Technology limiting drilling mud components' penetration in the formation. It includes drilling under minimal over-balance conditions in the system 'borehole-formation'. In such case it is justified to use drilling mud with surface- plugging water- or acid-soluble solid phase, ensuring minimal drilling mud filtration. This technology requires special materials - gel-forming and filtration reducing agents (biopolymers, materials based on modified cellulose and starch).
  2. Technology aimed at forming properties of the drilling mud and its filtrate which can prevent productive formation damage, i.e. restoration its initial permeability of oil. In this case drilling mud filtrate should have high surface-active and inhibiting properties. It should be noted that foreign oil and gas companies prefer the first technology, whereas the second one is mostly applied in Russia.

Application of biopolymer mud 'Burvis' for drilling through productive interval in Kotovskoye field resulted in 2.5 times production rate increase.

This mud is designed for drilling directional and horizontal borehole sections through productive intervals. Due to its resistance to polymineral aggression, 'Burvis' mud can be utilized for drilling horizontal sections from old existing wells. Clay-free drilling mud 'Burvis' has properties of biopolymer muds: it has a unique rheological profile, solids-carrying capacity; at the same time, it has properties of oil-base muds: excellent lubricating and surface-active characteristics; it has maximum compatibility with formation fluid, as there are no hydrocarbon components in its composition.

'Burvis' drilling mud was used for drilling directional and horizontal sections of boreholes through the productive intervals. Results of drilling through the productive intervals showed zero 'skin-factor' in two wells; in two other horizontal wells of Kotovskoye field 'skin-factor value was negative.

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