Study on Improving Recovery Factor by Horizontal Wells for Ultra Heavy Oil Reservoirs in the Western Depression of Liaohe Basin
After almost 40 years of exploration and development, Liaohe Oilfield has discovered proved heavy oil in place of over 1 billion tons, produced heavy oil reserves of about 0.74 billion tons, and the heavy oil production of Liaohe Oilfield has accounted for over 60% of its total production. Although heavy oil reservoirs have been put into full-scale development owing to the advances in cyclic steam stimulation technology and production practice, the cyclic oil-steam ratio has been near the economic limit in later cycles of steam stimulation, the reserves controlled by single vertical wells are low, the well conditions deteriorate, edge and bottom water encroaches, stable production is difficult to achieve, steam conformance is limited, and the recovery percent of reserves is low.
In order to improve the development efficiency of heavy oil reservoirs, a substantial amount of investigations has been conducted for horizontal well drilling technology and application at home and abroad, the research project of adjusting, planning and developing heavy oil reservoirs by using horizontal wells has been proposed, and the study mainly involves 3 aspects:(1) The study of residual oil distribution and the applied experiment of reservoir engineering design with horizontal wells for Liaohe heavy oil reservoirs. The target zones are technically analyzed in respects of fine structure description, reservoir description, sedimentary facies description, fluid property analysis, and well production conditions by using testing techniques of reservoir performance, fine interpretation of geophysical logging, reservoir engineering calculation with produced boundary and without grid, and numerical simulation. The areas rich in residual oil after many cycles of steam stimulation have been determined, a model of residual oil saturation between vertical wells has been built, and research trials of adjusting development program have been conducted for typical old heavy oil reservoirs with edge and bottom water and thin layers by using horizontal well technology. (2) Study on horizontal well drilling technology for heavy oil reservoirs in Liaohe Oilfield. In order to implement scale application of horizontal well technology to heavy oil reservoirs, meticulous study has been conducted for geological engineering design of single horizontal well in respects of reservoir distribution prediction, pilot hole optimization, well path optimization, hole structure optimization and completion optimization in accordance with the uncertainties of formation caving and lost circulation in heavy oil reservoirs. Dynamic tracking has been carried out on site to realized safe drilling. (3) Study on steam injection technique for horizontal section of horizontal wells in Liaohe Oilfield. In accordance with the characteristics of deep well, heavy oil and long horizontal section, the parameters of steam injection are optimized, the methods of steam injection and heat insulation are studied, thus improved steam injection and heat insulation effects, realized even injection in heavy oil reservoirs, and laid basis for improving recovery factor.
The following 4 cognitions have been acquired through the studies above:(1) Except the low pressure around wells experienced many cycles of steam stimulation, the other areas still maintain at initial pressure; the temperature increases differently at 30～50m from the well, and most of the rest part still keeps at original reservoir temperature; oil saturation between wells is basically about 65% of the initial saturation; oil viscosity reduces a lot at about 50m to around wellbore, and keeps almost at the original level beyond; (2) the structural high is less affected by bottom water coning and has rich remaining oil; the structure low is severely watered out as affected by both edge and bottom water. Horizontal well can effectively slow down the speed of edge and bottom water advancing, and improve the producing degree of reservoirs. (3) Geosteering is a guarantee for successful drilling of horizontal wells. For drilling in reservoirs with complex geological conditions, developed faults, big structural relief and drastic lateral change of reservoirs, geosteering must be applied. Geosteering performs real time follow-up according to the interpretation chart of the corresponding relationship between deviation angles and cutting lag time during build-up process, as well as the interpretation chart of the correspondence between different drilling assembly (screw drilling or rotary drilling) and lithology, rig time and drill pressure in different formations. It improves the accuracy of geologic tracking and ensures reservoir encountering rate. (4) The result of economic evaluation analysis indicates that, the average single horizontal well investment is 2.0～2.5 times of that of the vertical well; the average single well production is 2-4 times of that of the vertical; the average annual operation cost of single horizontal well is 0.5～0.66 time of that of the vertical; within the evaluation period, the average net present value of single horizontal wells is 23.8 times of that of the vertical. Horizontal well is an economic way for developing heavy oil reservoirs and can achieve better economic benefits.
This research study has obtained technical achievements in 4 aspects:(1) Drilling analysis and control technology for improving well path accuracy. The relationship chart of different drilling assembly (screw drilling or rotary drilling) corresponding to lithology, rig time and drill pressure in different formations has been established according to lab established interpretation chart of the corresponding relationship between deviation angles and cutting lag time during build-up process. Real time tracking has been conducted during horizontal well drilling, and the accuracy of well path tracking has been greatly improved. (2) Horizontal well drilling assembly has been optimized in accordance with the features of unconsolidated heavy oil reservoirs for safe and fast drilling, the capacity of cutting-carrying in horizontal section has been improved, and the drilling fluid system of hole cleaning has been developed for horizontal well. Special tools for horizontal well has been use to ensure that the lineal contact force between casing and sidewall distributes in a uniformly, slightly and gradually increasing pattern without abrupt change of peak value, and the maximum stress is less than half of the limit stress. Well path design is optimized to form gradual building up; the drilling fluid system of high temperature resistance and lost circulation prevention suitable to high temperature voided reservoirs has been used to solve the problem between hole stability and reservoir protection, thus ensured safe drilling and improved drilling speed. (3) J-shape horizontal well drilling technology has been developed for the problem of horizontal section cementation in heavy oil reservoirs with top water. For heavy oil reservoirs with top water, where the build-up section is unconsolidated glutenite and cementing quality is poor, a drilling method of reverse drilling from the stable mudstone below to enter the reservoir has been developed, i.e., J-shape horizontal well, thus improved cementing quality in highly deviated section. (4) The technology of reservoir engineering with horizontal wells has been developed for overall adjustment of old oilfield development after many cycles of steam stimulation. Geological model and grid system are constructed for pilot area through numerical simulation; the conformances of pressure and temperature are observed through basic data analysis and history match; the distribution of residual oil saturation is determined; and the lateral and vertical location of horizontal well, the optimum length of horizontal section, the optimum steam injection parameters are selected according to residual oil distribution.