China has an abundant coal resource, and the total amount of coalbed-methane resources is 36.81 10
The paper reviews practical applications of geosteering to horizontal well drilling and evaluates its efficiency for horizontal drilling. On Ust-Tegusskoye filed more then 30 horizontal wells were steered using 3D geological model.
One of key problems in geosteering process is limited ability to understand the location of the wellbore relative to the target object (productive layer, sand interval). The traditional set of data - structural maps build from seismic and pilot wells does not allow building fine structural capable to ensure successful drilling. But building the modeling workflow allows to simulate many possible locations of horizontal wellbore between top and bottom of reservoir, find the most probable location and minimize risk of unsuccessful drilling. Local update of the geomodels in the relatively small regions containing newly drilled wells helps getting production forecasts from flow simulator in reasonable time.
Recently large attention is paid to developing innovative approaches to the steering of horizontal wells.
The key requirement to the geosteering process today is landing well trajectory not only into pay part of productive layer, but into most efficient part of reservoir with minimal penetration of dense sub layers. For that the whole set of technological solutions exists ranging from optimization of pilot wellbore locations and finding best possible well profiles on geological and flow simulation models to the real time geosteering.
On Ust-Tegusskoye field the number of drilled horizontal wells grows every year: in 2009 there were 3 wells, in 2010 - 5 and in 2011 - 15. At the first half of 2012 9 horizontal wells were drilled (Figure 1). Minimal thickness of target layer for horizontal wells is 2.5 m. Maximum length of horizontal part is 832 m.
In spite of continuous improvements in drilling technology the problem of finding borehole position within top and bottom of target layer is still actual. Standard set of G&G information - seismic based structural surfaces and pilot wells does not allow building geomodel with reliability required for successful steering of horizontal well. Wrong positioning of well trajectory make cause it to deviate from target layer - so effective length of the well will be significantly reduced.
A three-dimensional discrete fracture network (DFN) model was developed in order to simulate the hydraulic characteristics of a granitic rock mass at Korea Atomic Energy Research Institute (KAERI) Underground Research Tunnel (KURT). The model used a three-dimensional discrete fracture network, assuming a correlation between the length and aperture of the fractures, and a trapezoid flow path in the fractures. These assumptions that previous studies have not considered could make the developed model more practical and reasonable. The geologic and hydraulic data of the fractures were obtained in the rock mass at KURT. Then, these data were applied to the developed DFN model. The model was applied in estimating the representative elementary volume (REV), the equivalent hydraulic conductivity tensors, and the amount of groundwater inflow into the tunnel. The developed DFN model can determine REV size for the rock mass with respect to the hydraulic behavior and estimate the groundwater flow into the tunnel at the KURT. Therefore, the assumptions that the fracture length is correlated to the fracture aperture and the flow in a fracture occurs in a trapezoid shape appear to be effective in the DFN analysis used to estimate the hydraulic behavior of the fractured rock mass.
The positional uncertainty about a point on a wellbore is commonly represented as an ellipsoid. The ellipsoid also accounts for the dimensions of the casing or open hole. Using this model, at any time the resulting uncertainty about a wellbore along its trajectory is a curved, continuous cone. To a good approximation, the intersection of the plane normal to a reference well with these cones can be represented as ellipses. This simple geometrical model has been adopted by various standards organisations to define minimum acceptable separation distances between well bores, for example the Norwegian NORSOK D-10 standard.
Because of mathematical difficulties, the existing methods for calculating the resulting separation factors are only approximations and may be either too optimistic or too conservative, particularly for ellipses with high eccentricities. The paper presents explicit equations for determining the exact condition where the ellipses touch, expressing the result as an expansion scale factor. Methods are presented for the expansion of either one, or both ellipses, together with implementation notes and other associated tools. The new algorithms are only marginally less efficient than the existing approximation methods and they can be used to increase the allowable proximity of two adjacent wells whilst satisfying the geometrical and probabilistic constraints. The examples included in the paper illustrate this.
The proposed calculation method is consistent with existing industry wellbore uncertainty models. Since the determination of the osculating condition is exact, the calculation is neither too optimistic nor too conservative. This paper is a response to discussions held at the SPE Wellbore Positioning Technical Section meeting on 3rd November 2011.
Micro seismic data and coring studies suggest that hydraulic fractures interact heavily with natural fractures creating complex fracture networks in naturally fractured reservoirs such as the Barnett shale, the Eagle Ford shale, and the Marcellus shale. However, since direct observations of subsurface hydraulic fracture geometries are incomplete or nonexistent, we look to properly scaled experimental research and computer modeling based on realistic assumptions to help us understand fracture intersection geometries. Most experimental analysis of this problem has focused on natural fractures with frictional interfaces. However, core observations from the Barnett and other shale plays suggest that natural fractures are largely cemented. To examine hydraulic fracture interactions with cemented natural fractures, we performed 9 hydraulic fracturing experiments in gypsum cement blocks that contained embedded planar glass, sandstone, and plaster discontinuities which acted as proxies for cemented natural fractures.
There were three main fracture intersection geometries observed in our experimental program. 1) A hydraulic fracture is diverted into a different propagation path(s) by a natural fracture. 2) A taller hydraulic fracture bypasses a shorter natural fracture by propagating around it via height growth while also separating the weakly bonded interface between the natural fracture and the host rock. 3) A hydraulic fracture bypasses a natural fracture and also diverts down it to form separate fractures. The three main factors that seemed to have the strongest influence on fracture intersection geometry were the angle of intersection, the ratio of hydraulic fracture height to natural fracture height, and the differential stress.
Simply put, the most significant finding of this research is that fracture intersection geometries are complex. Our results show that bypass, separation of weakly bonded interfaces, diversion, and mixed mode propagation are likely in hydraulic fracture intersections with cemented natural fractures. The impact of this finding is that we need fully 3D computer models capable of accounting for bypass and mixed mode I-III fracture propagation in order to realistically simulate subsurface hydraulic fracture geometries.
Copyright 2012, IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition This paper was prepared for presentation at the IADC/SPE Asia Pacific Drilling Technology Conference and Exhibition held in Tianjin, China, 9-11 July 2012. Contents of the paper have not been reviewed by the International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the International Association of Drilling Contractors or the Society of Petroleum Engineers, its officers, or members. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of IADC/SPE copyright.
On the basis of coal-bed methane particularity in China, the pressure, penetration and water-bearing are low. Generally, an extra straight well is needed for the coal-bed methane horizontal wells. And the straight well is connected to the horizontal
wells for the entry of rod pump etc to drain water and collect gas. For the connection spot of the vertical well and horizontal well is less than 0.5m in diameter, the connection goal can not be achieved by conventional wellpath measurement. Based on
the features of horizontal well, a suit of remote needle technique and relevant equipment is invented, which is used to guide the intersection between horizontal wells and the vertical well. The theoretical principle is rotating magnetic field orientation.
By processing the real-time collection data of rotating magnetic field, the distance and angular deviation of drilling direction can be calculated to locate the drill bit and the vertical cavity, and connect the horizontal well and vertical well.
In this paper, wellpath measurement and controlling model , method of remote needle technique, and theory of needle equipment were elaborated. This technique has been applied successfully to six horizontal wells in Chinese coal-bed methane
basin, such as Ordos basin. All intersections were finshed once only. In the field application, 60m distance away from the cavity can be precisely located for leading connection, the signal of magnetic field is distinct. By the improvement of the
model, the system possessees the capability of measuring well deflection to guide the nitersection in 3D. The success of applying needle technique to Ordos basin demonstrates that the established measurement model of needle can be used for the intersection of coal-bed methane horizontal wells, and positioning accuracy of needle equipment satisfy the field demand.
China possesses abundant coal resource in the world. The total amount of coal-bed methane resources is 36.81×1012 m3, which is roughly equivalent to the amount of conventional gas resources, ranked third in the world. The coal-bed methane
reservoirs in China generally have the characteristics of low pressure, permeability, and water content. Considering enhanced gas recovery and economic benefit, horizontal well and multi-branch horizontal wells of coal-bed methane are the
main development mode. Based on the particularity and drainage mode of coal-bed methane well, an additional vertical well is usually needed to connect the horizontal coal-bed methane wells(fig 1), which is to put down screw pump or rod pump for
drainage and exploitation. The remote needle technique and equipment is not only one of the key techniques to connect two wells, but also the essential drilling technique of multi-branch horizontal wells/U type horizontal well. Currently, the routine
control technique of drilling well orbit uses MWD to locate and control the drill bit. Drilling technique of coal-bed methane horizontal well is required to realise the intersection between horizontal well and vertical well. So a higher demand is raised
to measure and control the wellbore trajectory. For the target area of vertical well is a narrow rectangle frame which is 0.5× 4~8m, MWD measurement can not meet the need for monitoring and controlling the trajectory. In order to realise the
connection of the coal-bed methane wells, rotating magnetic field measurement method is introduced. Magnetic source transmitter is installed in the drill bit of horizontal well, so the rotating magnetic field is formed when the drilling string
rotates. Moreover, a signal acquisition system is fixed at the vertical well, which is used to collect the rotating magnetic field signal to caculate the distance and direction of deviation by the established magnetic field measurement model. The produced 4-3/4" romeote needle instrument applied to intersect several horizontal wells in Erdos coal-bed methane basin successfully.
Keshavarzi, R. (Young Researchers Club, Science and Research Branch, Islamic Azad University) | Mohammadi, S. (School of Civil Engineering, University of Tehran) | Bayesteh, H. (School of Civil Engineering, University of Tehran)