1. Field of the Invention
The invention relates generally to use of matrix acidizing in subterranean hydrocarbon formations. In particular aspects, the invention relates to techniques for helping to evaluate the effectiveness of matrix acidizing.
2. Description of the Related Art
Matrix acidizing is a stimulation process wherein acid is injected into a wellbore to penetrate rock pores. Matrix acidizing is a method applied for removing formation damage from pore plugging caused by mineral deposition. The acids, usually inorganic acids, such as fluoridic (HF) and or cloridic (HCl) acids, are pumped into the formation at or below the formation fracturing pressure in order to dissolve the mineral particles by chemical reactions. The acid creates high-permeability, high productivity flow channels called wormholes and bypasses the near-wellbore damage. The operation time depends on such parameters as the length of the wellbore, the rock type, the severity of the damage, acid pumping rate, downhole conditions and other factors.
Matrix acidizing is also useful for stimulating both sandstone and carbonate reservoirs. Matrix acidizing efficiency in removing the formation damage is strongly dependent on the temperature at which the acidizing occurs and weakly dependent upon the corresponding pressure. The acid temperature in the formation depends on the convective heat transfer as the acid flows through the formation and on the reaction heat transfer due to the acid-mineral reaction.
Convective heat transfer is the main mechanism for temperature change during acid flow through wormholes. The acid temperature in the wormholes may vary by as much as 10-20° C. (18-36° F.), depending on the initial temperature difference between wellbore and the formation. The acid temperature at the end of the wormholes, about 1-10 m (3.3-33 feet) from the wellbore, may increase by 1°-5° C. (1.8°-8° F.) above the formation temperature at those locations, depending on the injected acid volume.
Along a wormhole, the temperature changes over time as illustrated by
At distances further away than about 1 meter (3.3 feet) and at the advancing acid front region, the acid temperature increases from the well temperature to the formation temperature. This temperature increase is still due mainly to convection heat transfer. However, in the transition between the two temperature levels, the reaction heat transfer between the acid and minerals changes the temperature behavior by smoothing out the temperature change on one side closer to the well and by uplifting the formation temperature by about 1°-5° C. (1.8°-8° F.) on the other side, as
Methods for monitoring and evaluating matrix acid stimulations have long been investigated. Recently, distributed temperature sensing (“DTS”) technology has emerged as a tool for real-time data acquisition and interpretation for evaluating matrix acidizing performance. Although the main advantages of this technique (i.e., real time temperature data acquisition along the entire well and great sensitivity) are impressive, there are several major disadvantages as well. First, the DTS fiber is placed inside the coiled tubing string. Recording temperature data with a reasonable resolution assumes that the fiber has to stay immobile for the entire time needed for data acquisition. Second, as the DTS fiber is a multi-point temperature sensor (i.e., the fiber can record temperature data along the well at multiple locations), there is a significant amount of temperature data transmitted to the surface and being processed for all times and multiple positions along the well. Several solutions have been proposed in literature trying to circumvent these disadvantages. However, these proposed solutions are expensive and not reliable.
The present invention provides devices and methods that are useful for helping to evaluate the effectiveness of a matrix acidizing treatment. The present invention provides an alternative to DTS technology for matrix acidizing performance evaluation. In a described embodiment, an array of sensors is located at or near the end of the tool string. The sensors are capable of detecting an operational parameter associated with matrix acidizing. In preferred embodiments, the matrix acidizing operational parameters are temperature, pressure, flow rate, flow direction, gamma ray, etc., or any combination of the above. These sensors are disposed upon the outer radial surface of a matrix acidizing bottom hole assembly anywhere along the tool. The sensors are operably interconnected with surface-based signal processing equipment.
The sensor array is separated into a first set of one or more sensors and a second set of one or more sensors. Each of the sets of sensors is capable of detecting a matrix acidizing operational parameter at a particular location within the wellbore at different times. Therefore, moving the bottom hole assembly past a particular location at a particular speed will permit the first and second sets of sensors to detect the operational parameter at the same location at two different times. If desired, more than two sets of sensors can be used, which will permit the operational parameter(s) to be measured at a single location at multiple times.
In operation, the tool string and bottom hole assembly are disposed into the wellbore until the sensors are disposed proximate a formation to be acidized. In currently preferred embodiments, the bottom hole assembly is disposed initially located proximate the lower end of the formation or portion of the formation to be acidized. During acidizing, the sensors detect parameters such as temperature, pressure, etc. related to the acidizing operation in a static location and provide these readings to the processing equipment. If desired, the bottom hole assembly and sensors may be relocated within the formation interval during acidizing to perform acidizing in different parts of the formation. This permits the sensors to provided temperature and/or pressure data from different portions of the formation interval.
After acidizing is completed, the tool string and bottom hole assembly are removed from the wellbore. During removal from the wellbore, the sensors will continue to provide temperature and/or pressure readings to the processing equipment. In a preferred embodiment, the tool string and bottom hole assembly are removed from the wellbore at a predetermined rate of speed so that the first set of sensors will be adjacent a desired location within the wellbore at a first time and the second set of sensors is adjacent the same location at a second time. The desired operational parameter is first detected by the first set of sensors at the first time and then detected by the second set of sensors at the second time, thereby providing detections of the operational parameters at a single point at different times. The matrix acidizing monitoring system of the present invention can be used to provide multiple measurements of operational parameters at multiple points within the formation.
Processing equipment, preferably surface-based, will interpret the data provided. For example, the temperature detected at a particular location along the formation interval is compared at a first time and a second time to determine whether temperature at the location is increasing, decreasing or unchanged at the location. Changes in pressure at the location can be similarly determined. If pressure/temperature changes are detected at multiple points along the formation interval, the changes along the formation interval can be modeled to help determine the effectiveness of the matrix acidizing operation.
For a thorough understanding of the present invention, reference is made to the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, wherein like reference numerals designate like or similar elements throughout the several figures of the drawings and wherein:
In operation, acid is pumped down the tool string 18 and is injected under pressure through the matrix acidizing bottom hole assembly 20 into the formation 16. The injected acid will enter wormholes 24.
Radial passages 32 are drilled through the tool body 26 from the central axial passage 28 to the radial exterior of the tool body 26. A sensor array 33 is provided proximate the lower end of the tool string 18 and preferably upon the tool body 26 of the bottom hole assembly 20. The sensor array 33 includes multiple sensors 34 which are divided into two sets of sensors 34a, 34b. The first set of sensors 34a is axially separated from the second set of sensors 34b along the length of the tool body 26 by a length (“x”) (see
Electrical cables 36 extend from the sensors 34 to a conduit 38 that is disposed within the central passage 40 of the tool string 18. In a particularly preferred embodiment, the conduit 38 comprises a conductor known in the industry as tubewire, which can be disposed within the coiled tubing to provide a Telecoil conductive system for data/power. The term “tubewire”, as used herein, refers to a tube which may or may not encapsulate a conductor or other communication means, such as, for example, the tubewire manufactured by Canada Tech Corporation of Calgary, Canada. In the alternative, the tubewire may encapsulate one or more fiber optic cables which are used to conduct signals generated by sensors 34 that are in the form of fiber optic sensors. The tubewire may consist of multiple tubes and may be concentric or may be coated on the outside with plastic or rubber.
The conduit 38 extends to surface-based signal processing equipment at the surface 12.
In conjunction with the processing equipment 40, the first set of sensors 34a is operable to detect at least one matrix acidizing operational parameter at a first time while the second set of sensors 34b is operable to detect the same at least one matrix acidizing operational parameter at a second time that is after the first time. The difference between the first and second time is based upon the rate of movement of the sensor array 33 within the formation 16 relative to a particular point of interest.
According to an exemplary method of operation, the tool string 18 and bottom hole assembly 20 are disposed into the wellbore 10 and advanced until the bottom hole assembly 20 is proximate the formation 16 into which it is desired to perform matrix acidizing. If desired, packers (not shown) may be set within the annulus 22 in order to isolate the zone into which acid will be released. Thereafter, acid is pumped down the tool string 18 which will then flow through the nozzle 30 of the bottom hole assembly 20 and into the wormholes 24 of the formation 16. During acidizing, temperature and/or pressure is detected by the sensors 34 and provided to the processing equipment 40 at surface 12. During acidizing, the bottom hole assembly 20 might be moved from one location to another within the formation interval 17. Therefore, the sensors 34 will provide temperature and/or pressure readings from different locations within the formation 16.
After the acid injection is stopped at time (ts), the work string 18 is pulled out of the hole at a constant speed that can be calculated depending on the time difference (tf−ts) and the length of the stimulated zone along the well. Thus, the time tf may be the time that the matrix acidizing bottom hole assembly 20 has traveled the entire well interval of interest. The number of sensors 34 will be dependent on the accuracy of the data acquisition. For instance, a single temperature sensor may not be sufficient for temperature drop data interpretation, as any temperature difference recorded might be due to either axial flow (flow inside the annulus 22) or radial flow (flow between the wellbore 10 and a wormhole 24). However, multiple sensors 34 could accurately identify of a recorded temperature variation is due to axial flow or radial flow. At least two temperature sensors 34 should be installed sufficiently far away from each other such that they capture temperature differences due to radial acid flow. In particular embodiments, the minimum distance between two temperature sensors 34 is greater than the radial diameter of the wormholes. Thus, it is preferred that the sensors 34 are spaced apart from each other on the tool body 22 by a distance that is greater than the diameter of the wormholes 24. Theoretical calculations show that the minimum distance between two temperature sensors 34 should be between 4 and 20 meters (13-66 feet), depending upon the reservoir properties (porosity, permeability, wormhole size and shape, geothermal gradient, thermal conductivity, etc.) and well details (shape, dimensions, completion type, etc.). The method could be refined by adding temperature sensors between the two end sensors. Adding more temperature sensors in between increases the accuracy of temperature variation measurement. In addition to the temperature sensors, other sensor types could be used. For instance, pressure sensors could also be installed. Both temperature and pressure measurements are useful in accurately evaluating the matrix acidizing performance when they are coupled with a mathematical model that solves the classical energy flow equation inside the well:
where ρ is acid density, t and z are time and the curvilinear coordinated along the well path, v is acid velocity, u=cp (T−Tref) and h=u+p/ρ are the specific internal energy and enthalpy, respectively, cp is the specific heat defined at reference temperature Tref, and T and p are acid temperature and pressure. Note also that Q is the term that includes all other heat exchange effects, such as heat loss due to acid flowing into/from formation.
The inventors have found that using an array of single-point temperature and pressure sensors at the end of the tool string 18 and pulling them out of the wellbore 10 at a pre-calculated speed has major advantages over DTS technology. First, the acquired data volume is much smaller. This makes the data interpretation process faster and less prone to errors. Second, as the tool string 18 and single point sensors 34 are pulled out of the wellbore 10 after the acid injection has been stopped (at time t=ts), the operator brings the tool string 18 back to the surface 12 in a shorter time. A DTS fiber and coiled tubing must stay immobile until all data is recorded (usually until time tf) and then pulled out of the wellbore. Systems and method in accordance with the present invention permit the use of robust, durable conduits, such as tubewire/Telecoil technology. These advantages translate to lower operational costs for the matrix acidizing performance evaluation process when an array of single point sensors 34 at the end of the tool string 18 is used. After real-time downhole temperature and pressure data is acquired and interpreted, the acidizing performance can be visualized by knowing how much acid was injected where. This information is useful for understanding how the formation 16 was treated and if more acidizing is necessary to obtain expected acidizing performance.
Those of skill in the art will recognize that numerous modifications and changes may be made to the exemplary designs and embodiments described herein and that the invention is limited only by the claims that follow and any equivalents thereof.