Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
2001-01-29
2002-08-27
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C324S324000, C324S674000
Reexamination Certificate
active
06441618
ABSTRACT:
BACKGROUND OF THE INVENTION
The subject matter of the present invention relates to a method and apparatus for passively and continuously monitoring the status of seawater advance toward water acquifers near coastal cities by placing sensors, such as a particular type of electrode cable system, in the ground near observation and injection wells, transmitting acquired data back to centralized processing facilities, and, responsive thereto, subsequently mitigating the advance of the seawater into potable water acquifers which are situated near the coastal cities.
Coastal cities rely on groundwater from subsurface aquifers to meet all or part of their municipal water needs. In cases of historical overdraft, water is withdrawn from the subsurface aquifers at a rate exceeding the rate of natural aquifer recharge. As illustrated in
FIG. 1
, such overdraft results in a lowering of the water table in the aquifers and is accompanied by possible intrusion of seawater into the aquifer. The breakthrough of seawater at wells supplying the drinking water has severe long term consequences on municipal potable water deliverability. For example, Los Angeles (LA) experienced such overdraft in the first half of the twentieth century (the 1900s). As a result, LA subsequently created the Water Replenishment District (WRD) agency to define and enforce reduced aquifer pumping rates and mitigate the effects of seawater intrusion.
Various means exist to mitigate seawater advance. One of these methods is to recharge the aquifers from surface spreading grounds. Another method involves injecting inert gas or fresh water into the ground using special injection wells, as illustrated in FIG.
2
. The water injection method consists of injecting fresh water into the aquifer, forming a fresh water ‘mound’ in the local area around the well, to create a zone with pressure above the pressure in the seawater, thereby mitigating the landward advance of the seawater. As an example, since the 1950s, Los Angeles has constructed approximately 250 seawater barrier injection wells of this type thereby resulting in three lines of injectors. These three different lines of injectors are illustrated in
FIG. 3
, where the position of the water injection wells is shown by the adjacently connected circles.
In addition to the wells constructed for water injection (i.e., injection wells) as shown in
FIGS. 2 and 3
, a number of observation or monitoring wells (i.e., observation wells) are typically constructed in the vicinity of the injection wells. These observation wells are used to periodically measure the pressure (i.e., the hydraulic head) of the aquifer in the neighborhood of the injection wells, and for occasional sampling of water chloride (i.e., salinity) levels. This gives information about how efficiently the injection wells are limiting the seawater advance. In Los Angeles, for example, over 700 observation wells have been constructed along the three lines of injection wells (shown in
FIG. 3
) in order to monitor the position of the seawater wedge. In a typical municipal setting with seawater advance into the aquifers, the water authorities sample the chloride concentrations at three positions (top, middle, and bottom) of each water sand unit, as illustrated in FIG.
7
. This provides information about how efficiently the injection wells are limiting the seawater advance in each sand unit.
However, a need exists to passively and continuously monitor, in the observation wells, the status of the seawater wedge and the resultant seawater advance toward coastal city water aquifers as well as the status of the injected fresh water in the injection wells.
Depending on the application, various types of in-situ sensors have been employed in the oil industry for general reservoir monitoring. See the following “first reference” which discloses general reservoir monitoring: Babour, K. A., Belani and J. Pilla, ‘Method and Apparatus for Surveying and Monitoring a Reservoir Penetrated by a Well Including Fixing Electrodes Hydraulically Isolated within a Well’, U.S. Pat. No. 5,642,051, the disclosure of which is incorporated by reference into the specification of this application. In addition, such sensors have been proposed for leak detection, soil heating and temperature mapping. See the following three “second set of references” which disclose leak detection, soil heating, and temperature mapping: (1) Berryman, James G., Daily, William D., ‘Optimal joule heating of the subsurface, U.S. Pat. No. 5,325,918, the disclosure of which is incorporated by reference into the specification of this application, (2) Daily, William D., Laine, Daren L., Laine, Edwin F., ‘Methods for Detecting and Locating Leaks in Containment Facilities using Electrical Potential Data and Electrical Resistance Tomographic Imaging Techniques’, U.S. Pat. No. 5,661,406, the disclosure of which is incorporated by reference into the specification of this application, and (3) Ramirez, Abelardo L.; Dwayne A.; Daily, William D., ‘Using Electrical Resistance Tomography to Map Subsurface Temperatures’, U.S. Pat. No. 5,346,307, the disclosure of which is incorporated by reference into the specification of this application.
Consequently, in connection with the aforementioned need to passively and continuously monitor, in the injection wells and the observation wells, the status of the seawater wedge and the resultant seawater advance in addition to the status of the injected fresh water, there is a further need to utilize ‘special sensors’ in the injection wells and in the observation wells to perform the step of monitoring the seawater advance and the status of the injected fresh water. These ‘special sensors’ can be the sensors disclosed above in connection with the “first reference” or in connection with the “second set of references”. Alternatively, these ‘special sensors’ can be new sensors which are adapted for the above stated purpose of monitoring the seawater advance and the status of the injected fresh water.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to passively and continuously monitor, in the observation wells, the status of the seawater wedge and the resultant seawater advance toward fresh water aquifers located near coastal cities in addition to the status of any fresh water injected into the injection wells.
Accordingly, it is a primary aspect of the present invention to permanently install sensors in the ground in or around an observation well located near the injection wells in order to passively and continuously monitor the status of the seawater advance toward fresh water aquifers near coastal cities in addition to the status of the fresh water injected into the injection wells (hereinafter, the ‘monitoring step’).
It is a further aspect of the present invention to implement the aforementioned ‘monitoring step’ by utilizing sensors which have been used in the oil industry, such as the sensors used for general reservoir monitoring, and/or the sensors used for leak detection, soil heating, and temperature mapping.
It is a further aspect of the present invention to implement the aforementioned ‘monitoring step’ by utilizing sensors that are specially designed for use during the steps of monitoring the seawater advance toward fresh water aquifers near coastal cities and monitoring the status of fresh water injected into the injection wells.
It is a further aspect of the present invention to use oilfield related techniques/methods [that are currently being used in the oil industry to detect and record the existance of underground deposits of hydrocarbon (such as oil)] for the purpose of: (1) detecting the advance of seawater toward fresh water aquifers near coastal cities, and (2) detecting the pressure in the ‘mound’ of fresh water that has been injected into injection wells, the purpose of which is to create a zone of pressure above the pressure in the seawater for mitigating (i.e., slowing) the landward advance of the seawater toward the fresh water aquifers that are situated near the coastal citi
Babour Kamal
Delhomme Jean-Pierre R.
Howard Peter V.
Rossi David J.
Wijnberg Willem A.
Bouchard John H.
Schlumberger Technology Corporation
Snow Walter E.
LandOfFree
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