Conventional methods of well casing using multi-string coaxial structures when drilling wells in complex geologic conditions prove inefficient at depths 5-6 thousand meters.
Shafagat F. Takhautdinov، Nail G. Ibragimov، Khamit Z. Kaveev، Gabdrashit S. Abdrakhmanov، OAO Tatneft، Almetyevsk، Russia
Ravil R. Ibatullin، Izil G. Yusupov، Tatar Oil Research and Design Institute، Bugulma ، Russia
Conventional methods of well casing using multi-string coaxial structures when drilling wells in complex geologic conditions prove inefficient at depths 5-6 thousand meters. Ratio of well diameters on the well head and toe is as large as ten to one (10:1)، which testifies to serious shortcomings of coaxial well design. Besides، because of necessity of isolation of different-pressure formations and sloughing intervals with intermediate casing strings، that is، temporary casing of well bore walls to be able to drill deeper، consumption of tubulars and cement increases. Separate casing strings and liners with the length from 500 to 4000 m must isolate these formations even if their thickness is 10 to 100 m.
Intermediate casing strings used for isolation of formations incompatible under drilling conditions result not only in increased consumption of tubulars، cement، power، added labour and time، but also in problems caused by drilling with small size bits at high depths and certain limitations when workover during production cycle.
In the light of increasing shortage of tubulars and cement the problem of simplifying of well design with simultaneous improvement of formations’ isolation becomes the most acute in the world practice of drilling.
One more issue – recovery of casings damaged from the inside. The number of trips and the time of drill pipes’ rotation in casings increases sharply with drilling depth. Isolation of damages using additional casing string and drilling deeper with the same size bit is a challenge.
Besides، damage of production casing string caused by corrosion، workover jobs، etc. often takes place. Repair of the damaged interval to the point of original quality and without loss of the diameter hole – is another challenge.
These problems had been tackled by TatNIPIneft Tatar Oil Research and Design Institute. A team of researchers has developed unconventional technology and equipment which do not have analogues in world practice of well construction. The technology allows local isolation of formations using expandable (enlarged in the cross section) profiled casing pipes with their length commensurate to the length of trouble zone، without cementing and without loss of wellbore diameter.
The technology of local well casing employs steel casing made from a proprietary alloy that when profiled throughout the liner length will run freely through its own fully expanded internal diameter. Expandable underreamers enlarge the wellbore، through the problem-zone interval، to the external diameter of the initial nonprofiled casing string. Once the profiled liner is in place، pressure from drilling mud injection straightens and inflates the liner to the full working diameter of the wellbore، pressing it tightly against the underreamed formation face. The profile liner uses special seal، which is applied before running profiled pipes into a well.
The technology and equipment for local well casing using expandable profiled casing pipes have been applied in Russia since 1980. These developments have received 15 authors’ certificates of the USSR، more than 45 patents of the Russian Federation and 53 foreign patents، in that number of the USA، Australia، Canada، China، India، Norway، Japan، Germany، Great Britain، Mexico، Italy، France and other countries.
Initially this technology was intended for isolation of fractured-cavernous zones with difficult circulation loss where all methods of isolation، save intermediate casings، fail. The problem of lost circulation has been successfully solved. Profile liners are used for isolation of lost circulation intervals regardless of cavernosity of the borehole and rate of circulation loss.
This technology was used by Russian operating companies in the oil fields of Tatarstan، Bashkortostan، Udmurt Republic، Samara and Orenburg oblasts، where it allowed to exclude from the well design 9 5/8² (245 mm)، 8 5/8² (219 mm)، 7 5/8² (194 mm)، 7² (178 mm) intermediate casings and liners intended for isolation of cavernous-fractured zones with difficult circulation loss.
Even if one intermediate string or liner is excluded from the well design، the diameter of all previous casing strings decreases and، consequently، consumption of tubulars and cement falls، which results in reduction of wells’ construction time. For example، exclusion of 9 5/8² (245 mm) liner in Well No. 53 drilled on Utyaevskaya area enabled to save 111 tons of casing pipes، 104 tons of cement and reduce construction time in half from the project one .
Much more serious problem arises، when the trouble occurs abruptly، and the existing methods of trouble control fail، and the design of wells does not provide for running of liners. In such situations operators often abandon the well.
For example، “Orenburgneft” operating company spent 2.5 thous. hours on unsuccessful isolation of Well No. 2602 using existing methods of difficult lost circulation control. The company decided to drill a new well with a more sophisticated design. Using profile liner only 27 m long، the trouble on this well was solved for 76 hours . On many wells two and three profile liners were used without loss the design diameter of production string. To date oil companies in Russia have installed more than 1،000 of these expandable profile liners as an alternative to running intermediate casing strings in trouble zones.
Based on the gained experience and research studies “OAO Tatneft” has developed technologies enabling to solve a number of problems using expandable casing strings، namely:
? Adding a length to casing strings and liners in a well with retaining of the flush joint in the joined string. For example، on Well No. 160 of the Bratsk oil-and-gas exploration company for the first time in the world practice of well construction، the length was added to the 85/8² (219 mm) casing from the bottom، without loss of its inside diameter، and thick cavernous interval at a depth 2634-2663 m with severe lost circulation was isolated using profile liner with the length 29 m. Both wellbore and the lower part of the casing were reamed using special tools، and the upper part of the profile liner was tightly jointed to the reamed section of the casing. This well had to be redrilled، since 1500 hours and a large amount of cement had been spent without success for isolation job. After local casing of lost circulation zone by profile liner the well was drilled to the target depth without troubles .
? Interval drilling-in and casing of lost circulation sloughing zones. This technology was first tested on Well No. 202 of the Garaevskaya area، belonging to operating company “Samaraneftegas” [2،8]. Two profile liners were run in this well. The first profile liner isolated the interval 345-361 m، and the second profile liner، tightly connected with the first، isolated another trouble interval 360-368،5 m. Thus، the trouble zone was isolated using two profile liners without cementing and without loss of working wellbore diameter. Upon isolation of the problem zone، the operating company drilled the well to its final total depth of 2670 m، without further problems. The drilling operations were completed in 836 hr and 19 bit trips
? Isolation of water production. Fig. 1 gives examples of expandable profile liner installation to shut-off influx of highly mineralized water in Aleksinskiy horizon in the Romashkinskoye oil field; pressure in the formation exceeded hydrostatic pressure by 630 psi (4.3 MPa) .
? Technology and equipment for control of water inflows in open horizontal wells using profile liners. The technology was first used in 1997 on Well No. 11251. This well was 100% flooded. Two profile liners with the length 8 and 82 m isolated water production intervals. After successful isolation of water entry the well’s production rate increased to 18-20 metric tons/day [3،5].
? Casing of sloughing zones in inclined and horizontal wells. Profile liner with the length of 144.5 m was used to isolate sloughing zone entered at an angle of 68° in the horizontal well No. 39454 [4،11]. On Well No. 1623 “G” of Bastrykskoye oil field caving problems occurred after the rig had drilled the horizontal wellbore، that led to tools sticking and jamming. Production of the well was impossible.
According to the drilling program، the well was drilled with a 215.9-mm drillbit to the depth 1239،5-m and 168-mm casing string run (Fig. 2). The rig continued drilling the horizontal lateral with 149.2-mm drillbit to 1298 m and then completing the section with 144-mm drillbit to 1435 m. Well logs identified sloughing clays in the interval 1248-1252 m.
To isolate the sloughing-clay interval، a 17.6-m long profile liner with packer elements on both ends was run in the well on 89-mm drill pipe and set it in the interval 1237-1254.6 m. The upper 2.5 m of the profile liner was expanded and tightly pressed into the production casing، which required a 142-mm diameter expansion tool. The lower end of the liner set in the gauged hole and was expanded with a 140-mm expander. The profile liner successfully isolated the problem zone، with only a slight loss of the internal-hole diameter. The well continues producing oil with good rate and no problems have occurred .
Practically all wells where this technology was used، were drilled to the projected depth، regardless of the number of trips، which testifies to high reliability of the technology and equipment. For example، on Well No. 2602 after isolation of lost circulation zone with the profile liner 1167 m were drilled by rotary drilling in 1008 hours and 79 trips.
? Protection of permeable zones against bridging by drilling mud and cement slurry. On Well No. 3484 two producing intervals were cased immediately after their balanced drilling-in (Fig. 3). Production rate of the underlying producing interval is now two times higher compared to the adjacent wells . On four wells producing formations were cased off with the help of liners made of profiled pipes [6،10] without cementing and practically with the initial diameter (Fig. 4). Production rate of these well is 1.5-2 times higher compared to neighbors.
? Repair of strings with preliminary increase of the inside diameter of casing under repair by expandable milling tool (reamer). This is especially important for repair of casing strings in being drilled wells by thick-wall profiled pipes، since along with high reliability of repaired section possibility to continue drilling by the drilling tool of the same size (diameter)، which was used before casing repair is provided.
On well Genchai-2 (Lithuania) a fracture 22 m long was isolated، using profile liner، with wall thickness 8 mm (5/16²)، length 41 m، in an intermediate string dia 219 mm، the inside diameter of which was preliminarily increased by 8 mm using reamer. The well was drilled to the target depth without loss of hole diameter .
On Well No. 35 (operator “Yamalneftegasgeologiya”) in the interval 2265-2279 m 219 mm intermediate string was damaged due to abrasion by drilling tool. This interval was isolated by 14-m profile liner. After the repair the string was pressure tested with 14MPa، and the well was drilled to the target depth 4210 m by rotary method on 1.98 t/cu.m mud (16.5 ppg) .
In Well No. 51 of the Pechyora oil-and-gas exploration company 42.5-m profile liner was used to repair breaking in the 245 mm intermediate casing .
Expandable profile liner known as OLKS-216U with the length 34 m was used for repair of 245 mm intermediate casing on the Tyumen ultra-deep well SG-6 in the interval 3664-3698 m. After the job the string was pressure tested with 20 MPa .
In many wells profile liners with wall thickness 5 mm were used to close perforations in 5 3/4² (146 mm) and 6 3/5² production strings. After the job the strings were pressure tested with 10-15 MPa. Following pressure test the string was perforated against the producing interval located below isolated sections and the well was put on-stream. Similar works on casing repair in the process of drilling and production of wells were done on more than 60 wells. Besides، simple in design and reliable profile hangers for fixing of the liners both in the vertical and horizontal wells have been developed .
As a result of carried out presentations 14 foreign companies (from the USA، Egypt، Saudi Arabia، Italy، Iran، Iraq، China، Vietnam، and other countries) showed interest to the technology of local well casing، specifically، application of profile liners as an alternative to intermediate casing strings، for protection of producing formations against bridging with mud and cement slurry، shut-off of water production in horizontal wells، and casing of laterals sidetracked from production casing strings.
We applied the technology to Vietnam’s White Tiger oil field well No. 818WT، which had extremely difficult lost-circulation in the Lower Miocene and intense sloughing intervals. The well was suspended and shut-in for 3 years because of the continuous problem of the drillstring getting stuck. It was decided to case off lost circulation and sloughing zones with two sets of profile liners joined in the well. The operation was carried out in two stages (Fig. 5):
- The first stage profile liner isolated the most severe lost-circulation and washed out intervals from 3287 m to 3405،5 m (118.5 m). The liner isolated three severe washouts or caverns that were 80 m، 8 m، and 21 m thick، individually (109 m total). (See caliper log in Fig. 5).
- The second stage profile liner isolated five smaller washouts from 3405،5 m to 3496 m، which individually were 14 m، 8 m، 13 m، 11 m، and 10 m، thick (56 m total).
Casing off the Lower Miocene formations with the two profile liners، tightly joined with each other and to the 245 mm casing string allowed to raise the mud weight to 1.64 metric ton/cu m. This created a pressure differential on the profile liners of 19 MPa.
The operator continued drilling the 215،9 mm hole section of the well to 4010 m، entering the Upper Oligocene. The 194 mm liner was run and cemented in the interval 3210-4010 m with no problems. Since 2001the well is producing with the rate 34t/day.
Adaptation of the technology of local well casing to the Kupal oil field has been made after study of local conditions of well construction in Iran .
Differential pressure between Asmari، Pabdeh، and Gurpi formations is 3000 psi (24 MPa). Asmari formation is penetrated with the mud weight 63 psf (1،02 t/cu m) and isolated with the 7² casing (177،8 mm) to a depth of approximately 3772 m. The Pabdeh and Gurpi formations are drilled-in with the 6 1/8² (155،6 mm) bit. Mud with inert filling agents is used. Mud weight for the Pabdeh formation is 81،5 psf (1،3 t/cu m)، for the Gurpi formation - 97-104 psf (1،56-1،67 t/cu m). These formations are isolated to a depth of approximately 4124 m with the 5² (127 mm) liner. The Bangestan reservoir، which is the primary target، is drilled with the 4 1/8² (104،8 mm) bit to a depth of approximately 4400 m with the mud weight 82،5 psf (1،32 t/cu m) and completed open hole. But drilling with the 4 1/8² (104،8 mm) bit poses a lot of difficulties and problems when workover during production cycle. In case local well casing technology involving profile liners، is applied for the Asmari formation isolation، it will make possible to drill the Bangestan reservoir with the 6 1/8² (155،6) bit، and thus، avoid problems associated with drilling with the smaller size bit.
Based on technical requirements of NIOC، a special threaded-welded modification of profile liner has been developed suited for application in conditions of high differential pressure and high temperature in trouble zones. Besides، design of this type of profile liner considers another requirement of NIOC – impossibility of welding at the wellhead after the Asmari formation has been drilled out.
In case field tests of equipment are successful، the subject on set up of Joint Venture for introduction of local well casing technology in the Iranian oil fields may be negotiated.
Local casing equipment has already been shipped to Egypt ، Italy ، and China for future works in these countries.
A plant has been set up in Tatarstan producing profile liners on a wide scale.
At present major preoccupation of researchers involved in improvement of local casing technology is to reduce time of isolation jobs، so that drilling crew could run and set profile liner per one trip. Besides، to meet the requirements of our foreign partners، we develop new standard sizes of profile liners.
This technology has gained world recognition in the petroleum industry. In 2001 the technology was awarded two gold medals at international exhibitions، at the IV International Salon of Industrial Property “Archimedes-2001” ( Russia ، Moscow )، and at the American Inventions Show IMPEX XVII (May 2001، PA، Pittsburgh ).
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