A major contribution on a micro scale

Richard J. Zinno, Weatherford Wireline, shows how a simple monitoring methodology is improving the precision of tracking microseismic events during hydraulic fracturing.

Microseismic monitoring, the detection and mapping of naturally occurring and very small seismic events in the reservoir, is one of the core technologies that helped crack the code in developing shale gas plays over the past decade. The technique, which is an evolution of earthquake seismology, uses geophones to track the minute seismic events induced during hydraulic fracturing to accurately pinpoint and track the growth of subsurface fracture patterns.

By gaining a sharper view of the fracture network, operators can be more selective as to which stages to avoid (i.e., stages with one of more natural faults) and which to exploit for the highest production gains. In this way, microseismic monitoring gives the operator an alternative to the imprecise and frac-intensive ‘pump and pray’ approach to shale resource development.

Since the first microseismic map was created for an operator in Barnett Shale in 2000, many different techniques have been developed, each promising a unique perspective to accurately pinpoint the fractured zones in the reservoir that have the highest production potential.

Selecting the correct microseismic technique is often dependent on the particular geological makeup of the formation one is trying to monitor. The oft-repeated refrain that ‘more is better’ - that the detection sharpness and accuracy of the measurement will be improved by simply placing more geophones in the array - usually falls short in the field.  A closer examination may show that a simpler array using fewer geophones with more precise sensors, coupled with a sophisticated analysis methodology and skilled petrotechnical processors, will provide a higher level of monitoring accuracy.

Sharpening the view in China’s shale plays

A shale gas field in China’s Sichuan Basin proved an excellent testing ground to compare the efficiency, accuracy and level of detailed reservoir information provided by several different microseismic monitoring techniques. Rock formations in this region of the Sichuan province have routinely displayed minor faulting and natural fractures, which were observed to alter the stimulated fracture network in horizontal wells. This has resulted in altered drainage patterns and lower productivity from these wells.

A 2012 well stimulation from an adjacent well pad experienced difficulties in pumping operations. Weatherford conducted downhole microseismic monitoring in real time, which allowed many on-the-fly alterations of the stimulation plan to successfully complete the operation. A small fault in the underlying limestone seemed to be the source of the pumping difficulties, which drastically altered the flow of the injected slurry. A major goal of head-to-head comparison testing is to identify the microseismic monitoring technique that would help the operator design the optimal frac job to avoid such faults in future wells.

Figure 1. An overview of the well site.

Dueling downhole arrays

The operator conducted a field trial designed to compare the efficacy of two competing downhole microseismic acquisition methods. A ‘zipper frac’ stimulation of two horizontal shale gas wells was monitored with an array of microseismic monitoring technologies. The real-time evaluation would help the operator determine which method serves as the best guide to adapt the stimulation procedures to geological conditions, thus improving the outcome of the well completion. 

The first downhole method favors placing more geophones in the array, in as many as 20 - 40 levels, which purports to improve the detection level and accuracy of the measurement. This method also uses fibre optic telemetry, which quickly brings large volumes of data to the surface for analysis - another factor that is seen as a clear improvement for microseismic monitoring in some circles.

The drawbacks to this methodology include a lack of precision in the measurement, coupled with a migration algorithm that is essentially a ‘black box’ micro-earthquake location technique, and no way to do any quality control with the analysis. The result: one ends up with mapped locations with a lot of built-in error and uncertainty in them.

Another downhole microseismic methodology championed by Weatherford calls for a simpler architecture with fewer geophones in the array, each containing more precise sensors. This method also applies the ‘gold standard’ of analysis methodology, based on the Geiger Simplex earthquake location. Finally, skilled processors are employed who can apply and understand the algorithms and the input waveforms to provide a precise location for a microseismic event. This results in much less error and more detail in the output maps.

Expanding the effort

In 2013 the development effort was expanded with three wells drilled on an adjacent pad. Curvature analysis of a 3D surface seismic survey showed that more structural complexity would be present along portions of the new wells. Given the significance of the faulting in the previous well, elaborate microseismic monitoring arrays were deployed in adjacent wells, in shallow monitor wells, and in a large surface pattern above the wells.

Real time mapping was performed from all arrays by multiple teams of geophysicists from different vendors. The results of the mapping contributed to a much more complete picture of the effect of these bands of natural fractures on the drainage area of the new wells. The mapping also allowed the stimulation procedure to be altered such that difficulties were avoided and the effectiveness of the stimulations was optimised.

Figure 2. A comparison of microseismic maps provided by the Weatherford array (upper left) and the borehole microseismic effort (lower right). The Weatherford array has twice the number of events detected in a frac that is staying close to the perforations. The Geospace array data has wide error 'arcs' of data scattered about the map. The increased level of detail and accuracy in the Weatherford array has enabled the operator to study and exploit the important relationship between the presence of natural fractures and large differences in created stimulated rock volumes.

Comparison conclusions

The microseismic technology comparisons provided several conclusions, some of which confirmed the commonly accepted relative strengths and weaknesses of the acquisition methods. For example, the commonly held views that surface arrays lack the depth resolution and ‘map view’ accuracy of borehole arrays positioned in or near the reservoir depth was confirmed in the real time results. This lack of resolution was observed by both large surface geophone patterns and shallow-well, near-surface patterns. Shallow borehole arrays were also unable to detect microseismic activity at reservoir depths.

These comparison tests also disputed the commonly accepted view that surface arrays may provide greater areal extent than borehole seismic arrays for mapping induced microseismic source locations. In this instance, the detection range of the borehole arrays - particularly the Weatherford array - was in excess of 1800 m. The borehole array also imaged microseisms that originated further from the treatment wells than either the surface or shallow well arrays.

The comparison of the two downhole arrays broke with common industry assumptions about which array geometries and which seismic sondes provide the most detection range and accuracy. Both systems used the same processing software to analyse their data, removing that variable from the comparison. 

It was clear that the detection range from the 8-level Weatherford array was superior to the detection range of the 30-level Geospace, fibre optic augmented array. Furthermore, the 8-level array allowed the detection of smaller magnitude microseisms, a larger number of mappable events and more reliable microseism locations.

Head-to-head comparisons of downhole instruments in a test well environment showed a greater sensitivity in the Weatherford sondes, particularly at higher frequencies, and greater vector fidelity as a result of that high frequency content. This improved signal quality enables superior microearthquake location accuracy and superior useable detection range. It is believed that it this higher quality of recorded signal that is the primary differentiator between competing arrays, rather than an assumed advantages of number of sondes in the array.

Adapted by David Bizley

Read the article online at: https://www.oilfieldtechnology.com/drilling-and-production/11032014/a_major_contribution_on_a_micro_scale/