Testing and completing deep reservoir in the state of Kuwait is challenging due to high contrast in formation pressure over the 4,000 feet section (13,500 ft to 17,500 ft). Recently, a new structure located in the North-Western part of Kuwait area was drilled and tested with deep drilling rig. Formations in this deep structure have very low porosity and permeability with naturally fractured reservoir. During short term testing, wells produced oil and gas with unexpectedly very high H2S(20%-35%), the highest H2S content in any reservoir so far discovered in the state of Kuwait. Since the reservoir was rated as HPHT and sour, it was a challenging experience to test all the wells safely and successfully. Unfavorable conditions such as high pressure, high temperature, high H2S and CO2 content require special equipment, tools and treatment to test and complete exploratory wells.
The targeted formations were perforated using deep penetration Tubing Conveyed Perforating (TCP) guns, stimulated with emulsified/retarded acid, tested with Drill Stem Testing (DST) tools and sampling was done. Meticulous planning and testing strategies could overcome all challenges and high sour reservoir was successfully tested safely without any incident. The wells were flowed only for short period due to high sour nature of the reservoir fluid and safety reasons. However, data collected during short term testing helped in understanding production capability, fluid and reservoir characteristics which is vital for delineation/development of the new field. Methods developed, data obtained and expertise gained in testing and completing high sour wells will be useful for the future development of sour gas reservoir.
Aim of this paper is to describe the actual challenges encountered and lessons learned during testing and completion of hostile environment wells. This paper also aim to present techniques and approach adopted to address the operational risk and HSE issues during perforation, Coiled Tubing, stimulation, well testing and sampling operations.
Exploration of deep reservoirs has become very important to produce high pressure gas and light-oil to meet the country's gas requirement.
Jurassic carbonate reservoirs in Kuwait fields are deep, low permeable, High Pressure High Temperature (HPHT) and generally contain sour volatile oil/condensate and gas. Formations of these deep reservoirs are tight and naturally fractured carbonate reservoirs. Najmah(NJ), Sargelu(SR), Upper Marrat, Middle Marrat and Lower Marrat are the five formations proved as a hydrocarbon bearing reservoirs. Najmah/Sargelu formations are tighter than Marrat formations.
Najmah Formation consists of Limestone at the top part and Shale to the bottom. Sargelu Formation consists mainly of shale and limestone. Najmah & Sargelu (NJ-SR) are naturally fractured reservoirs. So, fractures play an important role in production from NJ-SR formation. Marrat Formation consists mainly of limestone, occasionally with streaks of anhydrite at the top and with streaks of shale at the bottom of this formation. The core analysis of Middle Marrat formation shows that dolomitization plays major role for matrix porosity enhancement which is responsible for good reservoir character.