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The Oil Mines at Wietze and Pechelbronn

Written by Jeff Corwith

 

 

 

Introduction

 

After collection of oil from surface outcrops and seeps, mining of oil is arguably the oldest means of exploiting hydrocarbon resources. Heavy oils, used to caulk ships, were hand dug from shallow pits in the Middle East several hundred years BC. (Lyman et al, p 565). In recent times, two oil fields in Europe have become notable for their development through subsurface mining methods. These fields are Wietze, the birthplace of the USE's oil industry in the 1632 milieu, and Pechelbronn, located in the Alsace not far from the borders with present day Germany.

 

Wietze Oilfield

 

Oil seeps and "Teerkuhlen" (tar pits) in northern Germany were known and used from the middle-ages. The first "oil well" was drilled on behalf of the Hanoverian government from the tar pit at Wietze in 1859, the same year that E.L Drake is credited with starting the modern oil industry in North America (Kauenhowen, p. 472-474). Historical production rates for Wietze (Reeves, p 1578) are shown on Figure 1.

 

 

Mining operations started up in 1917, as denoted by (E) on the figure. By the time the field was shut in 1963 (G), some 16 million barrels of heavy oil and 2.7 million barrels of light oil were produced. Of these volumes, 2.6 million barrels of light oil and 9.5 million barrels of heavy oil were produced from wells, with the remainder produced from mining operations. Further discussion of well productivity and potential development scenarios for the USE (by drilling) will be the subject of a second essay; for now, the remainder of this essay will focus primarily on mining operations.

 

Mining at Wietze

 

Mining operations for oil recovery at Wietze were initiated in 1917, based on successful efforts at Pechelbronn (Bloesch, p 413). Two shafts were sunk in 1917 for the drainage of oil into the mine galleries (Rice et al, p 407). While production was initially achieved by seepage from the reservoir into the mine galleries, the mining operations were later changed to actual extraction of the oil-bearing sands themselves. The mined sand was brought to the surface where it was washed to extract the oil (Torrey p 589).

Underground extractive mining of oil sands uses methods and technologies common to coal mining (Lyman et al, p4). Mining techniques such as "room and pillar" and longwall mining, with which the USE up-timers would be well familiar, can be employed to recover oil bearing sands.

While the exploitation method for Wietze was initially chosen to be the same as for Pechelbronn, the reservoirs themselves are significantly different. While the Pechelbronn accumulation is a stratigraphic trap, consisting of lenticular sand bodies interbedded with (impermeable) limestones and marls (Clapp, p 1098); the oil at Wietze is found in structurally complex, steeply dipping sandstone beds on the flanks of a piercement-type salt dome (Reeves, pp 1560-1561).

By the time mining operations were shut down in 1963, the mines had produced 964 thousand tonnes (6.8 million barrels) of oil (Zander). Taken over the life of the mine, average daily production corresponding to this volume was over 400 barrels per day.

It is worth noting that over 75% of the oil produced from the mines was through drainage rather than actual extractive mining of the sands themselves. In comparison, conventionally drilled wells produced 1.7 million tonnes (12 million barrels) of oil, nearly 80% of which was the 'heavy' oil. Ultimately, while oil mining was significant, the conventional production methods proved more viable, even for the shallower 'heavy' oil intervals.

 

Operational Challenges at Wietze

 

The oil extracted from the mined intervals at Wietze was found to be of heavy gravity, low in content of light ends such as gasoline (Bloesch p 416). Rice (p 307) reported that the miners at Wietze encountered very little gas released from the oil. Because of the low gas content, Wietze did not experience problems with fire danger.

The mining operations were not without challenges, however. Prolific aquifers are present in the shallow strata overlying the mined intervals. When the original shafts were sunk, it was necessary to freeze the area immediately around the shafts until they could be sealed off (likely with masonry) to prevent flooding the mine workings (Rice et al, p 302).

Additional problems arose from the nature of the sands themselves. The oil sands were reported to be "mushy" and unconsolidated, resembling oil quicksand (Rice et al, p 303). Bloesch reported that the mining operations were quite difficult and expensive due to measures required to keep the soft formations from collapsing the works (Bloesch, p 415). Late in the life of the mining operation, the mine was used for research on different recovery mechanisms (Torrey, p 589; Wiesenthal, p 1313). The ultimate result of the research showed that as much additional oil could be produced by various recovery processes using wells drilled from the surface as could be economically mined, leading to the eventual shut-down of the mining operations in 1963 (Torrey, p 589).

 

Exploitation of the Wietze oilfield in the Ring Of Fire world

When up-timers from Grantville start development of the Wietze field (Flint), one might expect that their natural predisposition would be toward startup of mining operations as soon as possible. However, there are a number of factors which should be given consideration in our speculation of how the USE oil industry would proceed with development of the Wietze field.

First is finding and delineating the oil deposits. It is unlikely that any person in the USE will actually know exactly where the oil deposits lie. True, there are reported oil seeps at Wietze, however these oil seeps apparently do not directly overlie the producing field, as the first wells drilled from the Wietze tar pit were unsuccessful (Kauenhowen, p474).

Furthermore, unless an up-time essay detailing the subsurface geology of Wietze is available; there will be a significant amount of time spent just building an understanding for the complex structure and stratigraphy present in this area. One can imagine the tensions between Quentin Underwood and his geoscientists. ("I'm paying you friggin' rock doctors to find my oil, not to drill dry holes and collect data for your Imperial University dissertation!”, bellows Underwood). The geoscientists would need to study drill cuttings from each well in order to create cross sections and maps for the various stratigraphic horizons. Each well is like a piece in an incomplete puzzle. Whether the well is dry or oil bearing, each set of data helps to put the three dimensional image into focus.

As exploration / appraisal wells are drilled, some will strike oil zones. It would follow that these would be put into production as soon as facilities are available to process the oil. Given that the Wietze field has been described as being very complex, there will still likely continue to be a significant fraction of dry holes drilled after the initial discovery. (The actual fraction of dry holes or "non-commercial shows" drilled is unknown, but likely to exceed one third of the total wells drilled after the first discovery well. Further investigation will be the subject of discussion in the subsequent essay dealing primarily with drilling and production issues.)

It likely would follow that management would grow increasingly impatient at the rate of discovery and production which would ensue. Eventually, as the geoscientists can obtain data from increasing number of wells, their understanding of the field will improve the odds of any given well finding the oil reservoirs.

Having a good idea of where the oil bearing zones may lie, the USE oil industry may choose at this point to commence construction of a mine. From a modern oil industry perspective, this is likely not the most cost effective approach, as the early development wells will be quite productive as they will penetrate undepleted reservoir intervals. In our time frame, the Wietze field had been under production for four decades by the time mining operations were initiated in 1917 (Kauenhowen, p 476).

Nevertheless, let us suppose that the management of the USE oil industry takes the decision to commence mining operations. Early commencement of mining operations will mean that the Ring of Fire miners will face two challenges not faced by those in OTL.

The first challenge has to do with pressure. The mine shaft will be at effectively atmospheric pressure, but the oil zone will be initially pressurized at or above the regional hydrostatic gradient (typically 0.45 psi/foot, or 9.9 kPa/meter). Unless the reservoir had been significantly depleted through years of production, accidentally mining blindly into the oil reservoir could have dire consequences.

The second challenge has to do with the gas originally dissolved in the crude oil. While Rice reported that there was very little gas associated with the mined oil, his observations were made after decades of production had the opportunity to deplete the reservoir pressure. The miners will need to take precautions against both the presence of explosive gases and reservoir pressure.

Neither of these challenges are insurmountable. The miners at Pechelbronn successfully developed exploitation methods to mitigate their gas problems; such could also be employed by the Ring of Fire oil miners at Wietze. The main shafts and drifts would be ideally situated outside of the oil bearing zones, within impermeable rock. Under one scenario, drifts (galleries) would be excavated paralleling the oil bearing intervals. Horizontal boreholes, with slotted pipe set in the reservoir zone, would be drilled at intervals along the drift to recover oil and gas and depleting the pressure and dissolved gas until the region of the reservoir rock is safe to enter with a cross drift for excavating the oil sands themselves. In our time line, this practice had been recommended to the miners of Wietze by a mining engineer from Berlin (Rice et al, p. 308)

Safety against fire and explosion danger from methane (and heavier) gases will be critical in the operations. The Wietze miners will need to have access to the Grantville miners' gas detection equipment. When a section of reservoir is accessed, it will first have to be cleared for gas content, and then periodically re-checked to ensure that the atmosphere stays well below the lower explosive limit.

The structure at Wietze is exceedingly complex, with steeply dipping beds below an unconformity, a major thrust fault plane dipping at 20-30 degrees north through the field, and ...