Recent technical and logistical challenges in the Gulf of Thailand have driven the development of a new approach to lightweight cementing. The industry has evolved from numerous and complicated dry cement blends being used on 45-day well constructions to the present minimum liquid additive, lightweight, single slurry approach used on 5-day wells. As nearly all wells are now slimhole monobores, the cement job has become one of the most important operations in the life of the well. The absence of packers or other isolation devices means that the cement job functions as a primary pressure barrier and is the only technique to isolate the numerous sands in each well. This combined with the logistic issues of fast offshore batch drilling exact a premium on cementing products and the entire cementing process itself.
For offshore applications it is advantageous to use an all-liquid system so as to avoid relying on mud pits to blend mixwater, to avoid the confusion of multiple dry blends in the field and to have flexibility to make last minute changes in slurry design. Recently a new approach to lightweight cementing using Multifunctional Liquid Additives (MLA's) was developed so that on-the-fly mixing is possible with only three to four streaming additives. MLA's have reduced what used to be a complicated set of dry blends to a few liquid additives. By simply adjusting the cement:water ratio and MLA concentration, the same base slurries can be tailored for nearly any scenario. This approach has been used recently in PTTEP's Arthit gas field to construct a 1.60 SG. production string slurry at 190°C. The same additives work for shallow gas where temperatures are over 150°C cooler. Because the systems are so robust, slurry charts have been implemented such that the rig can decide what slurry to pump based on a given bottom hole circulating temperature (BHCT) and desired thickening time.
This paper illustrates the various ways lightweight slurries using MLA technology have created a new approach to offshore well construction in The Gulf of Thailand.
PTT Exploration and Production Public Company Limited (PTTEP) operate Thailand's largest gas field, Bongkot, and adjacent Arthit field. The concessions are located at 600 km South of Bangkok and 203 km off the coast of Songkhla province (Fig. 1). Water depth over the fields varies between 75 and 80 meters. Most of the wells in the fields are high temperature wells with the average thermal gradient being approximately 6°/100 meters of vertical depth. This equates to 180°C at 2,500 mTVD as an average. Pore pressure gradient is generally hydrostatic in the early life of a field, but tends to drop below hydrostatic over time. However, virgin pore pressures in the Arthit Field have been as high as 1.50 SG in deeper reservoirs. Gas is typically trapped in sand lenses, along faults, in multiple levels of sand-shale sequence.
Over the years there have been several types of well design implemented in these fields - among them; conventional wells (with standard size geometry), tubingless wells, ultra-extended reach wells, deep target wells, and horizontals. Wells have even been drilling into the basement. At present, the standard wells are based on slimhole architecture, which have been extremely successful in the GOT. A typical slimhole well is constructed with the following three sections.
11"1/2 hole × 9"5/8 casing, 500 mTVD / 500 – 550 mMD. Conductor Sharing technique is used for development wells, two wells are drilled from one slot that has been split by partition plate on 12-slot platform. Casing drill-in is used if wells are appraisal well drilled without platform.
8"1/2 hole × 7" casing, 1,200 - 1,600 mTVD / 1,200 – 2,000 mMD. After surface section was cased, the well will be drilled to reach top of commercial reservoir, 7" casing is run and cemented.
6"1/8 hole × 3"1/2 completion, 2,600 – 3,000 mTVD / 2,600 – 4,000 mMD. Reservoir section is drill with small 6"1/8 bit. After electronic logging, 3"1/2 completion is then run and cemented in place, which will be perforated and gas is produced through. This makes cement as an important part of well completion.