Mud losses while drilling is a serious problem in the Mumbai Offshore, affecting drilling time and well productivity adversely and increasing the well cost. Mumbai Offshore oil field is a multilayered reservoir. The producing reservoir of the fields is mainly comprising of fractured carbonate which is highly porous with various dimensions of porosity i.e. micro, meso and macro. The intensity of fluid invasion is very high due to the pattern and lithological characteristic (cavernous, vugular, fractured formation). Initially, wells were drilled using conventional or non-conventional drilling fluids as the reservoir pressure was the main concern and to overcome of pressure and other drilling problems, mud weight was to be kept high or equal to the formation pressure.This field has been producing for the last 35 years and there exists differential pressure depletion across the field. Loss circulation during drilling appears to have become more severe with increased directional drilling activity and reservoir depletion. There are also number of wellbore instability incidents like stuck-pipe, tight hole and cavings evidenced at shale intervals especially when there was a drop in hydrostatic head of wellbore annulus due to mud losses in carbonate reservoirs. This problem in the carbonate reservoirs of Western Offshore fields of India is well known since long time and several remedial practices have been tried in the past with partial or no success. This problem has further escalated with depletion of reservoir pressures as well as increased directional drilling activity.
Presently, the formation pressure has declined to sub hydrostatic level. Sub hydrostatic pressure and mud loss problem due to above stated lithological characteristics in such reservoir may cause wellbore instability, differential stuck ups, sometimes blow out condition even in sub hydrostatic formation. It has also been observed that especially in slim-hole where lifting of cutting is not proper, then a chance of mud loss is relatively high.
If lost-circulation zones are anticipated, preventive measures are taken by treating the mud with lost-circulation materials (LCMs). The pressure in some of the sub-hydrostatic reservoir layers are as low as 5ppg MWE or less. Presently the Losses are usually controlled by-
a) Use of Sub hydrostatic Drilling Fluid
b) Using System LCM Concentration
c) Multiple Use of LCM Pills
Mud loss rate in some of the fields have been recorded in the range of 100 BPH – 400 BPH or even more. Many of the time such formations are being drilled under severe to total mud loss conditions. These Mud Losses can be categorized under different groups:
a) Partial Losses / Seepage Losses (10-20 BBLs/Hr).
b) Moderate losses (20-60 BBLs/Hr)
c) High Losses (60-140 BBLs/Hr)
d) Severe Losses or Total Loss (>150 BBLs/Hr)
In western offshore, most of the carbonate reservoir is the major contributor towards oil & gas production. The reservoir pressure in some of the producing fields have been depleted. Therefore, in view of continuous pressure decline in producing reservoir, serious drilling & completion challenges have been faced. The problem is likely to aggravate with time. Therefore, an appropriate drilling and completion strategy needs to be worked out on a fast track basis. Broad expectation is a cost effective environment friendly, non-damaging to formation drilling fluid/ remedy for drilling loss prone depleted reservoir having fractured, cavernous and vuggy carbonate formation.
II. Boundary Conditions
• The solution should control the partial and total losses both under static and dynamic conditions.
• Should be applicable in vertical, deviated as well as in horizontal wells in offshore fields.
• The solution should not have any detrimental effect on the producing formation and on productivity and life of the well.
• The solution needs to be economically viable.
• The application and handling of innovation should be ONGC’s HSE compliance. The LC-50/LD-50 data should be in accordance with Ministry of Environment guidelines.
• The Loss Control solution needs to be simple, easily applicable, and without any major modification in the Rig specifications.
• The method/technique should be applicable to the well/reservoir up to 4000 m.
• The method/technique should be applicable to the reservoir temperature range of 600C to 1500C.
• In case the solution is a “LCM pill”, the pill type can be in the category of –
a) Particulate type
b) Thermosetting type
c) Dilatant Type
d) High Fluid Loss type
The combination of pills type is also a possible solution for the LCM to be effective. The LCM Pill needs to be compatible with ONGC Mud System (SOBM, KCl PHPA etc). The LCM pill is to be reservoir friendly and acid soluble.The Loss control method is to be rig and environment friendly and needs to be pump-able through Drill string (with motors/RSS assembly, PBL sub, tubulars and bit having nozzle greater than 14/32”). The present pill concentration with RSS/ Motor is restricted to 40 PPB.