No abstract

The Triassic Baikouquan Formation reservoir in the Mahu Sag, Junggar Basin is characterized by tight sandy conglomerate oil reservoir with long drilling cycle and great fracturing difficulty. The factory drilling and fracturing techniques of horizontal wells for the Ma131 Well Block of Mahu Oilfield have been determined by regional geological characteristics research and optimization scheme design. The key technologies for horizontal well drilling speed improvement were explored by designing small three-stage wellbore structure, customizing the PDC bit properly, optimizing drilling tool combination, and selecting the drilling fluid with inhibition and plugging ability. The factory reservoir stimulation technique carried out horizontal well hydraulic fracturing with small well spacing for two sets of beds, and adopted bridge plug + clustering perforation staged fracturing and slick water + gel composite fracturing technology. 12 wells in a drilling pad were arranged in the demonstration area with the depth of 5000m, the distance between adjacent sections was 150m or 100m, with the horizontal section length of 1800m. Batching drilling operation pattern was adopted, with 3⁃well drilling for each of rig. Rapid inclining prevention drilling through vertical sections adopted PDC bit + screw + MWD, with high rotary speed and high flow rate. In the build-up section, radical drilling parameters was used to improve the top drive rotary speed and release the torque. In the horizontal sections, rotary steering tool + low speed high torque screw was adopted. The application of this technology achieved good results, with average drilling cycle of 48.3 d and average ROP of horizontal section increased by 77%. The fracturing construction adopted high pad fluid ratio, high sand ratio and high pump rate. More than 10,000 m3 fluid was injected in a single well with high-intensity proppant injection. The factory operation was carried out in the order of construction from the two sides to the middle well with microseismic monitoring. Factory fracturing with small well spacing generated stress interference and improved the fracture reconstruction effect. Higher shut-in pressure, more microseismic events and large-scale stimulated reservoir volume had been carried out in horizontal wells fractured later. The factory drilling and fracturing techniques of horizontal wells for the Ma131 Well Block have achieved the goals of improving drilling speed and volume fracturing, which provided technical support for the efficient development of the tight sandy conglomerate oil reservoir of the Mahu Oilfield.

The long-term continuous exploitation makes it increasingly difficult to maintain sustainable hydrocarbon production onshore in Dagang Oilfield. The shelf sea area of Bohai Basin with an average water depth of about 2.5m is identified as the main take-over battlefield, which needs to be developed by extended reach drilling (ERD) due to the lower loading capability of subsea thick silt and the inaccessibility of drilling rigs onshore or offshore. This is one of the major ERD campaigns implemented independently onshore in China.Favorable achievements have been obtained during the 4-year ERD play. Four extended-reach horizontal wells with the horizontal displacements larger than 4000m have been successfully completed by integrated application of pseudo-catenary trajectory design, casing floating running technology, compound drilling method, real-time ECD monitoring, highly inhibitive drilling fluid, optimized hole cleaning operation and enhanced drag/torque reduction technology. The maximum horizontal displacement to vertical depth ratio reaches 3.92. Many other ERD records have also been made in China onshore, building basic foundations for much larger-scale of application of ERD in Bohai Basin. Meanwhile, many new challenges have also emerged, such as the wellbore instability in highly deviated intervals, inappropriate design and optimization of PDC bit, instable bottom hole assembly (BHA), difficult control on well trajectory for the long angle-holding section, etc. This calls for technical innovations to address problems associated with previous ERD activities.This paper focuses on the technical achievements as well as the lessons learned from the ERD play in recent years, with its purpose to explain where we are and what to do next for further research and development of ERD technology in China. Helpful case histories and recommendations will also be presented.

Managing lost circulation has always been a serious challenge in the oil industry. This situation becomes further complicated when shale formation is involved. Particulate lost circulation materials (LCMs) have been used to prevent the drilling fluid from entering fractured, cavernous, or high-permeability formations for many years. However, existing LCMs are inefficient in terms of size and application methods in solving severe-to-total losses in shales due to the uncertainty of particle size distribution (PSD) in drilling fluids. This study focuses on an experimental method optimizing the PSD of solid particles in the drilling fluid at high temperature and pressure. An orthogonal experiment of L9 (3)4, including four factors and three levels, was designed to investigate the relationship between the PSD of solid particles and filtration loss. Results indicate that the orthogonal design method contributes to the optimization of the PSD of solid particles in the drilling fluid and reduce the test time and experimental workload during the drilling fluid process in the design stage. In addition, a polynomial relationship between the cumulative filtration loss and D90 or D50 is verified experimentally in this study. This study will remarkably benefit the drilling technologists in resolving the issue of lost circulation.

Reservoir shale has the characteristics of bedding planes, strong anisotropy, and strong imbibition ability under ultralow water saturation, which has an important impact on wellbore stability of horizontal wells. Therefore, the physical and chemical properties of reservoir shale were analyzed, and a mechanics model of wellbore stability was established considering the elasticity and strength anisotropy. The wellbore stability was analyzed considering the anisotropy parameters of formation elasticity and strength and imbibition conditions. The results show that the anisotropy of elasticity and strength under the condition of imbibition are important parameters for evaluating the stability of reservoir shale borehole. Considering the interaction between fluid and reservoir shale, the weakening of the strength of the bedding structure is the reason for wellbore instability of reservoir shale. Bedding planes and microfractures provide rapid imbibition channel, and the imbibition volume increases, which further causes fracture expansion and strength weakening, and then leads to wellbore instability. Multistage effective plugging of bedding, microcracks and macroholes during drilling is the key to maintain the stability of shale wellbore.