Recognition of Capillary Seals in Hydrocarbon Accumulations Using SP - - PowerPoint PPT Presentation

Recognition of Capillary Seals in Hydrocarbon Accumulations Using SP Logs

Stephen P. Cumella, William F. Woodruff, and Andre Revil

Characteristics of SP Anomalies Discussed in This Talk

• SP anomalies are absent in the first

wells drilled in a new area of a basin.

• As wells are hydraulically fractured and

produced, SP anomalies begin to appear and tend to increase in magnitude as more wells are drilled.

• Steps in the SP typically occur at
• rganic-rich intervals such as coals,

carbonaceous shales, etc.

• The SP anomalies are much bigger in

areas of Mamm Creek field that have been completed with large water- volume hydraulic fracs compared with adjacent areas completed with smaller fracs. Completion intervals shown in pink

Rollins

Price Coal MVRD

AAPG Bull., 2017

• +

Membrane potential Liquid-junction potential

SP Electrochemical Potential

Schlumberger, 1991

Figure 1a sketches a clay mineral with different crystalline planes. All these planes contain a density of electrical charges when in contact with water due to a variety of charging mechanisms. This charge is locally counterbalanced by a charge of opposite sign in the so-called electrical double layer. As a result of this electrical double layer there is a net (generally positive) amount of electrical charge in the pore water. Under a pore-fluid pressure gradient, the pore water flows from high pressures to low pressures through the interconnected pore space (Figure 1b). This flow drags the excess of charges contained in the pore water (Figure 1c) generating an electrical current density called the streaming current that is negative in an upstream direction and positive in a downstream direction.

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Electrokinetic Potential Results from Water Movement through a Formation

MOC #27-07 Jonah Federal From Dean Dubois

Piceance Basin, Mamm Creek Field SP Anomaly Caused by Streaming Potential

• Well on left drilled in 1994,

undepleted sands have minor SP deflection, high neutron-density crossover

• Well on right drilled in 2005,

depleted sand has big SP anomaly due to streaming potential, no crossover due to invasion past depth of investigation of neutron and density curves 392 ft

Borehole Pressure Greater Than Formation Pressure Negative “Normal” SP Response Formation Pressure Greater Than Borehole Pressure Positive “Reverse” SP Response

Streaming Potential

Electrokinetic SP Response Resulting from Differences in Borehole Vs Formation Pressure

+

• Mud filtrate invades formation

Overpressured formation water flows into borehole

• +

SP Log SP Log Serra (1984) p.78

Bartberger and Pasternack, 2015

Bartberger and Pasternack, 2015

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Cumella, Woodruff, and Revil, AAPG Bull, 2017

SP Difference between Upper and Lower Williams Fork Increases with Time

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Piceance Basin, Mamm Creek Field SP Cross Section with Pressure Data

Map view diagram illustrating the mechanism for the negative SP anomalies in the Piceance

• Basin. a). Blue arrows show water movement towards early-vintage gas wells shown in gray and

the electrical field that results from production-driven water migration in the formation; b). Infill gas wells shown in red, drilled in areas of negative electrical potential. In the Mesaverde we observe a classical SP response in early (gray) wells with negative anomalies developing

• ver time in later infill wells (red). Water movement within the formation results in

electrochemical potential in which the upstream areas become negative and the downstream areas (producing wells) become positive.

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Capillary Seals

Key Points from Papers by Cathles and Others

• Cathles, Revil, Shosa, and other authors published a series of papers over a decade ago on

pressure compartmentalization caused by capillary seals that form when two fluid phases exist in interlayed fine and coarse sediments.

• Shosa and Cathles (2001) propose that the leakage of water through capillary seals is zero,

even in geologic time.

• Capillary seals are durable; they should re-heal if ruptured by faulting or fracturing.
• Gas/water interfacial tension is about twice oil/water interfacial tension, so gas will form

stronger capillary seals than oil.

• The amount of gas or liquid hydrocarbon required to form a capillary seal can be quite

small, since only the areas immediately adjacent to fine-coarse interfaces need contain a non-aqueous fluid (Price Coal example).

• The effect of even a small amount of gas and oil on the thermal conductivity of

sedimentary rocks is dramatic; for a constant heat flow, the thermal gradient can easily be doubled.

• The invulnerability of capillary seals to rupture could allow gas to be trapped in basins for

very long times, perhaps explaining the phenomena of basin-centered gas.

From Shosa and Cathles, 2001

In this experiment, a 0.5 meter tube contains five sediment packages consisting of an 11 mm layer of 45-micron coarse silt, a 13-15 mm layer of 2-micron very fine silt, and an 11 mm layer of 45-micron coarse silt. The remaining volume of the tube was filled with 0.5 mm coarse sand. Water with dissolved CO2 was driven through the tube at various flow rates; the pressure drop across the tube was measured at each rate. The system pressure was lowered below the partial pressure of CO2 for this solution and a separate CO2 gas phase formed and the capillary force between the gas and the liquid stopped the

• flow. Results of the experiment showed that the total pressure drop across the tube increases linearly with the number of 2-micron layers. The flow

blockage by individual fine layers is thus cumulative. The best estimate of the capillary pressure drops per 2-micron layer is 34.1 psi.

Shosa and Cathles 2001

From Revil and others, 1998 Abnormal Pressure at Shallow Depths in an ODP Borehole in the Mediterranean Sea Caused by Capillary Sealing

Model for SP response due to electrokinetic potential resulting from water flow within pressure compartments formed by capillary seals

The first well drilled in an area with no previous Mesaverde production has little SP response and the SP log drifts to more positive values with depth (e.g., the well on the left). After Mesaverde production is established, producing wells produce water in addition to gas. The water movement creates an electrical current resulting from the movement of positive charges in the direction of water flow. Water movement will always be away from unproduced areas and these areas will develop a more negative charge with continued water production in adjacent

• wells. Water movement is largely confined

within pressure compartments that are isolated by capillary seals formed adjacent to coals. This cross section depicts higher volumes of water flow within some compartments resulting in more negative SP response in a recently drilled well (the well on the right). The blue arrows indicate water flow and the length of the arrow is indicating the volume of water flow within each compartment (longer arrow=higher volume

• f water flow).

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Cumella, Woodruff, and Revil, AAPG Bull, 2017

Examples of SP Anomalies from the Piceance and Other Basins

Increase in SP Anomaly with Time, Wells in Section 20, 6S-92W, Mamm Creek Field

SP Colorfill Scale

Rollins Price Coal Top Gas MVRD

Gamma ray in track 1, SP in track 2.

• 170

Rollins Price Coal Top Gas MVRD

Gamma ray in track 1, SP in track 2. SP Colorfill Scale

• 170

Piceance North Parachute-Red Point Area

MVRD Price Coal Rollins

San Juan Basin Pictured Cliffs-Fruitland SP Cross Section

Pictured Cliffs Lewis Shale Fruitland Coal

Pictured Cliffs

Anadarko Basin Granite Wash SP Cross Section

Desmoines

Cross section by Ben Funderburk, Forest

Capillary Seal?

Pressure-depth plots for five 30-mile x 30-mile areas, dotted lines show gradients ranging from hydrostatic to lithostatic. Pressure data are taken from a) Al-Shaieb and others (1994), green symbols; b) mud weights from IHS Energy (2010), red symbols; c) bottom-hole pressures from IHS Energy (2010), blue symbols; and d) drillstem tests from IHS Energy (2010), brown symbols. Area A is underpressured as shown by separation between hydrostatic gradient and right edge of drillstem pressures. Area B is underpressured, but less than area

• A. Area C is normally pressured as shown by absence of gap. Areas D and E are overpressured where data exceed 0.5 psi/ft. Only a

small fraction of drillstem pressures are valid in situ pore pressures, most are less than the true insitu pore pressure. (Nelson, USGS OFR 2011-1245).

Uinta Basin, Natural Buttes Field SP Cross Section

TGR3 Wasatch

Mesaverde Castlegate

Uinta Basin, Natural Buttes Field SP Cross Section SP Anomaly Occurs near Coal

Uinta Basin, Brundage Canyon Field SP Cross Section Cross section by Dan Berberick, Ute Energy

DJ Basin Niobrara SP Cross Section

A B C

Codell

Ft Hays

DJ Basin Niobrara SP Cross Section

Summary Points

• In most basins and play types where we have looked, we have found SP anomalies.
• Typically, SP anomalies are absent in the first wells drilled in a new area of a basin and

are common in later wells.

• These anomalies are due to electrokinetic potential, not electrochemical potential.
• These SP anomalies are located at capillary seals, which are formed in strata adjacent

to hydrocarbon sources such as coals and organic-rich shales.

• We suggest that the SP anomalies are the result of water flow within compartments

formed by capillary seals.

• These capillary seals may be critical to the occurrence of unconventional hydrocarbon

accumulations.

• The capillary seals form pressure barriers that may help explain the pressure

distribution observed in overpressured accumulations such as the Piceance Basin.

• Because capillary seals act as insulators, temperature logs can reveal the presence of

capillary seals.

• Our suggested mechanism for what may be causing the SP anomalies may be wrong,

but the SP anomalies are very really and are probably very significant.

• The methodology for quickly identifying these anomalies is simple and anyone with a

few minutes and a large number of digital SP logs can determine if they exist.