Oil & Gas News
Tuesday, 21 April 2009
Drilling Mud Optimisation for Horizontal Offset Wells
Petrov N.A., Korenyako A.V., Davydova I.N., Komleva S.F
When drilling pilot-operational well 1557/22 in the Sugmutskoye field, polymer-clay replenished mud was used as a basis in a horizontal section of the well in the same way as conventional directional wells in Noyabrsk region.
Chemical treatment of the mud included the following materials and chemicals:
Bentonite powder PBMA
The mud was modified via additional treatments with polymers and lubricants (LUB-167 and graphite) and a comprehensive drilling mud surfactant - SNPH-PKD-515. The latter contributed to inhibition (hydration of clay) and surfactant properties. Carbonate heaver was added to increase the mud weight, with Pipe-lax additive in reserve in case of any sticking problems. Following that, a filter (perforated sub) was installed, FSG-146, in a horizontal section. Before sealing the space above the sub with a PDM-146 packer, the drilling fluid, contained in the horizontal section, was displaced with low-concentrated HCl with some cationic surfactant - 0,5-1,5% wetting agent IVV-1.
Oil reservoirs have been developed successfully for quite a while through directional drilling where the horizontal section of a wellbore targets the pay zone. Horizontal wells allow increased oil recovery from a formation due to the following factors:
Drilling and production of horizontal wells began in the Noyabrsk region, particularly in Sugmutskoye field in the early 1990's. This field is in an area of prioritized use of natural resources. Oil deposits were tapped within the field in formation BS 9-2 which represents a complex sand-shale reservoir with West Siberia' generic interlaying of hygrophilous sandstones and mudstones. The roof of the payable formation in the center of the deposit is between 2708 - 2714 m. Waters of BS 9-2 and AC 7 are of the calcium chloride type, with the salinity ranging from 12,10 - 17,03 gr/l. The average formation temperature at the water-oil contact of the formation BS 9-2 is 88C, with a formation pressure of 28.1MPa.
The Senoman deposits of the Sugmutskoye field are not productive. From experience of drilling directional production wells, there is a chance to impair well bore stability, which may result in pipe sticking at intervals above 2700 m. Drilling the interval below the surface casing shoe usually requires utilization of salt-resistant fluids of a polymer-clay nature.
Efficient transportation of cuttings and good carrying capacity of mud are critical factors when drilling wells with a horizontal bottom-hole. Effective transportation of solid particles can be achieved via imparting adequate energy. This provides a turbulent flow with a high velocity. If there is a higher concentration of cuttings in the mud due to a high drill rate, it may exceed the sand-lifting ability of the mud. Therefore, mud velocity in the annulus is regarded as one of the main parameters of bore cleaning. At a very high velocity of turbulent flow, most cuttings may be carried away with the flow. At low velocity, of the stream cuttings may accumulate on the low side of the wellbore wall resulting, eventually, in a cuttings pad on the bottom.
The task of defining what velocity is required to create a turbulent flow in the annulus is difficult. Moreover, turbulent flow should not be created when erosion-sensitive formations are the case, or if the pump is at limited capacity. Rotation of the drilling tool initiates a spiral flow, which in turn assists in cuttings removal and prevents the formation of new layers and sand drifts.
Acceptable levels of well cleaning (with mud) can also be achieved at moderate velocities. For example, at the laminar flow when mud flow characteristics are accurately defined.
Mud flow characteristics (rheology) in a turbulent flow are not linear. Here, there may be concurrently low viscosity and have appropriate carrying capabilities.
High carrying and the thixotropic properties of fluids precludes from the inner sedimentation of solids. At the same time such fluids should contribute to a minimal hydraulic resistance during the course of drilling. This favorably influences the performance of a drill bit.
Cleansing quality at the laminar flow is more sensitive to high viscosity at low gel strength (GS), which should be proportional to the yield point (YP). There is a certain dependency between these values and the quality of cleaning. Experience shows that the YP should be in the region of 30-40dPa, plastic viscosity should stay within 15-20 MPa*s, with relatively low gel strength (gel strength (1) not higher than 10dPa or gel strength (10) not higher than 40dPa). Favorably, funnel viscosity should be kept within 20-30sec.
Adjustment of mud flow characteristics can be done by altering the concentration of the colloidal clay components of mud solids and high-molecular polymer compound.
Stability of bore walls is achieved through adequate mud weight as well as the selection of inhibiting (water repellant) and filtering properties. In particular, fluid loss of polymer-clay mud should be ultimately low, ca. 3-5 cm3/30 min using the BM-6 meter.
The inhibition properties of the filtrate influence the size and properties of the cuttings particles. It is practically good precedure to have a 4-stage mud cleaning programme; in order to enhance performance of the cleaning equipment (mainly a shaker, centrifuge and settling tank) one should use coagulants and flocculants.
The optimal content of solids within the drilling fluid fed into a well bore is 20-22%, including a colloid clay component percentage of 1.6-1.8%. There should be virtually no sand (or it should not exceed 0.5%).
Drilling horizontal sections requires the usage of lubricants that minimize the friction factor between the filter cake and the drill pipe. Drilling fluids successfully used in conventional directional wells serve as a basis that certainly requires modification before drilling a horizontal section.
Polymer-clay types of mud are widely spread in drilling operations in the Noyabrsk region. The chemical composition of drilling mud is mainly the following:
Therefore, the chemical treatment of drilling mud when drilling out from surface casing to 2700 m is performed in accordance with operations procedures applicable for directional drilling. Chemical treatment has a number of features within the highly deviated drilling sections below the intermediate casing shoe (when the building angle is as high as 90%), as well as when drilling a horizontal well in productive formation. In particular, it does the following:
For engineering support of the pilot operational well No. 1557/22 at the Sugmutskoye field (1997) the following procedures of drilling mud preparation and conditioning were planned:
The ejector unit prepares the clay slurry by mixing high quality clay powder, with a density of 1020-1030 kg/m3. The clay-based mud is then mixed with chemicals KMTs-700 or CMC (USA) using 200-250 kg of dry chemical per 100 m3 of drilling fluid. The drilling mud is treated with the chemical CMC, whether in a dry condition or dissolved in water. If it is necessary to replenish the volume of circulation liquid then a water based solution is used, consisting of the chemical CMC and Saipan (Japan) in a ratio of 2:1. The mixture is diluted in a clay mixer on the basis of 2-3 kg CMC, 1,0-1,5 kg Saipan and 1 cubic m of water. Following that, a chemical called Polikem D (Kem-tron, USA) is added to this water composition on the basis of 0.5kg to 1 cubic meter of water.
KMTs-700 chemical is used for lowering fluid loss and adjusting the flow rheology of the mud. Saipan, an Acryl polymer, also reduces fluid loss value and, in addition to this, facilitates well bore stability thanks to its encapsulating (inhibition) effect. The high viscosity of - Polikem D allows a regulated flow rheology of the mud, showing flocculating and inhibiting properties.
In order to achieve both inhibiting, hydrophobic and surfactant properties, the chemical SNPH-PKD-515 was used, which is a composition of non- ionogenic and cation-active surfactants. Complex surfactant blends well with almost all chemicals currently used in the region. The water solution SNPH-PKD -515 is then used to condition drilling mud in a water-agent ratio of 3:1, and is thinly jetted at the pump suction point. Mud conditioning should be started from the interval below the surface casing.
Conditioning with LUB-167, a lubricant from Kem-Тron in the USA, should also start when drilling out from the surface casing and to ensure the concentration of thechemical is maintained in the mud.
Lubricant LUB-167 allows for the efficient reduction of torque, and resistance to the motion of the drilling pipe (drag). Since LUB-167 is a chemical that blends well with almost every chemical used in the region, it can be used in any water-based drilling mud.
For better lubricating mud, it is recommended to additionally use 2-3% graphite for the volume of mud in circulation. Moreover, for well engineering support it is important to have a 200kg reserve of Pipe-lax, an anti-sticking additive made by Kem-tron.
To achieve a higher density of drilling mud to meet procedural parameters, a carbonate heaver should be applied. In order to neutralize Ca2+ions when adjusting pH, 1.5kgs of sodium carbonate per 1 cubic meter of mud can be used.
As the length of the horizontal bore increases, and prior to drilling completion, drilling mud is treated with combined water solution KMTs-700 and Polikem D in a ration of 6:1. The water solution is added evenly throughout circulation cycle.
The operational parameters of drilling mud at intervals between 2700 - 2900 and 2900 - 3100m vertically are presented in the below table.
The sequence of conditioning drilling mud at the above intervals is as follows:
Between 2700 - 2800 meters: drilling mud is treated with the following:
up to 1 ton of KMTs-700 chemical
500 kg of Saipan
100 kg of Polikem D.
5-7 tonnes of preliminary hydrated bentonite powder, type PBMA, then needs to be added for every 100 cubic meters of mud.
During mud circulation 2tonnes of LUB-167 lubricant need to be added, 600 kg of SNPH-PKD-515 surfactant and 20 tonnes of carbonic heaver.
Between 2900 - 3000 meters: At this depth, drilling mud will need to be additionally treated with the following:
400 kg Saipan
400-800 kg of KMTs-700 chemical
up to 50 kg of Police D
up to 700 kg of LUB-167
and up to 600 kg of SNPH-PKD-515.
When drilling a horizontal bore, drilling mud must conditioned with:
2 tonnes of KMTs-700
Up to 700 kg of polymer Saipan
up to 100-150 kg of Polikem D in combination with up to 2 tonnes LUB-167, Up to 1 tonne of graphite
up to 600 kg of SNPH-PKD-515
It is quite logical that during the construction of the first horizontal wells at each field, as a rule, the number of unreliable sections not covered with casing is reduced to minimum.
Thus, for example, before drilling the horizontal section of a well bore an intermediate casing is run into the hole in order to cover the unreliable well bore lengths. Accumulating experience in the construction of sloped and horizontal wells and increasing the quality of drilling mud will ultimately lead to drilling without intermediate casing, which allows for significant savings in capital expenditure.
Indeed, construction of a well with a horizontal offset bears certain risks associated with the potential loss of well bore stability from the moment of drilling-in, untill casing running and cementing. The probability of stability loss of rock is most likely at high inclination angles.
However, rock properties across the geological profile of Sugmutskoye field at high inclination angles has not been studied sufficiently enough. Therefore, at key well No. 1557/22, the plan was to run in an intermediate casing at a maximum vertical depth of 2680m in order to achieve a minimum or zero probability of stability loss in the open hole interval when drilling horizontal section of a well bore.
Since running intermediate casing with a diameter of 245 mm results in a considerable cost increase for horizontal well construction, new data obtained during key-well drilling has enabled operators to optimize the design of other horizontal wells at the Sugmutskoye field.
During the drilling of this well, a new completion program was in effect. The program provided for setting a 146-mm non-cemented casing in a horizontal section with pre-fabricated and installed perforated subs (FSG-146). When running the casing, subs play a role of casing, ensuring the hole circulation through a shoe; and later on, when the well is producing, the subs provide hydrodynamic communications between the productive formation and the well. Before sealing the casing annulus above the horizontal section with packer FSG-146, clay mud in the horizontal bore section is displaced with acid.
The composition of acid solution includes 4-6% of concentrated HCl, with the addition of 0.5-1.5% of water-soluble surfactant cathion (wetting agent NBB-1). Particles of carbonic heaver dissolve under the influence of the acid solution, which in turn restores reservoir permeability. At the same time, the cathion-active additive inhibits the corrosion process on the casing during acid solution injection, inhibits deacidification and hidrophobize pore space at the bottom-hole formation zone, which facilitates cleaning of the bottom-hole formation zone from "impurities" during well flow stimulation.
Because a horizontal well bore section has a non-cemented perforated sub, the quality of tailing-in is, in fact, entirely dependent upon the quality of drilling mud.
In conclusion, when drilling wells with a horizontal offset, special care should be taken when selecting the type of circulation fluid and its composition, as well as when setting an ongoing monitoring process over the parameters of enriched drilling mud. It should also be possible to easily adjust its properties should the need arise.