A study dealing with formation damage and damage control for horizontal wells drilled in the Shengli and Dagang (both in East China) sandstone formations is reported in this paper. The factors influencing the productivity of horizontal wells were quantitatively analyzed. The results show that formation damage is an important factor affecting productivity, compared with other factors, such as reservoir thickness, horizontal well length and anisotropy ratio of permeability, for the low permeable formations with permeabilities less than 80 md. It was also found that the effect of formation damage on production rate is constantly much higher for a horizontal well than that for a vertical well drilled in the same formation under the similar drilling conditions.
To minimize the formation damage during horizontal drilling, three less-damaging drilling and completion fluids (a newly developed polymer mud, SN-1 oil emulsion mud and MMH mud) were used in these horizontal wells. The design criteria, formulation, properties and applications of these fluids were discussed. For each mud system CaCO3 was used as a bridging agent. The particle sizes of CaCO3 were selected based on the average pore sizes of the target formations. It was observed that the three mud system had provided excellent formation protection with much less skin factors and much larger percentages of regained permeability as compared with other commonly used drilling and completion fluids. The oil production rates of most horizontal wells are three to five times higher than that of their adjacent vertical wells, which is highly related to the effective prevention of formation damage.
It is well known that the major purpose of horizontal well drilling is to increase the contact area with reservoir exposed in the well and thereby enhance well productivity. In recent years, the horizontal well technology has been developed quickly in China, and more than fifty horizontal wells with various radii, most of which were located in Shengli field, were drilled in the 90's. It should be noted that the difficulty and cost of drilling and producing increase with the augmentation of well productivity. One of the major concerns is that the drilling-induced formation damage may be more pronounced for a horizontal well than that for a vertical well in the same formation from the point of view of the production loss (i.e economic loss) although it is generally considered that the horizontal well will likely be more tolerant of formation damage than the vertical well since the horizontal completion interval length is much longer.
Thus, in order to give a full play to the economic benefit of horizontal well technology, it is necessary to know the characters of damage accounted during horizontal drilling and to develop techniques to minimize the extent of damage caused by drilling muds. The formulations, performance and effectiveness of these drilling muds were discussed. All this work was in connection with the horizontal wells drilled in Shengli and Dagang low-permeable sandstone formations.
The test procedures to evaluate drilling-induced damage are similar to those provided in Ref. 2. Both the values of skin factor and regained permeability are mainly used as the measures of damage in this work. Flow tests through field cores were conducted at ambient temperatures. All the core samples were taken from vertical wells in the same formation and saturated with synthetic formation brines. These tests involved comparing the final (regained) permeability, after treated with drilling fluids, to the initial undamaged permeability.