SCP is taking another new step to unblock the massive but challenging gas reserves of the Yanbei field for the resource holder and our collaboration partner, Yanchang Petroleum. Yanbei’s valuable resources are buried 3.5 km under the rolling hills of northern Shaanxi.


At these great depths, there exist fossilized 260-million-year-old river systems stacked on top of each other after repeated cycles of glaciation. Over the eons, the pressure from these overlying layers crushed buried vegetation into coal which, in turn, released natural gas which collected in the porous sandstones of these ancient riverbeds. Now it is our duty to drill down to produce this resource to the surface to benefit China’s energy supply.



图:A general model of the original depositional environment




图:Examples of what the original rivers that made S1-H8 looked like


But it takes technology investment and skill to locate these rock formations. And it is not enough just to drill a well into this rock, as it is so impermeable that no gas can flow through it unless we also create a network of small fractures spreading out from each well into the reservoir. These fractures open the rock pores and free the trapped gas so it can flow to the surface.


This approach has been proven effective to develop a group of rock layers called S2-3, which contain nearly half of Yanbei’s gas. To fracture S2-3 layers and release the gas, we pump into each well, a slurry consisting of gum-thickened water and tiny ceramic grains at high pressure. Our engineers use advanced engineering software to determine the pumping pressure and fluid specifications. After a sufficiently large fracture network has been created, we stop the pumps and allow the natural rock pressure to try to clamp the fractures closed, but the closure process stops when it encounters the solid ceramic grains. Billions of these grains “prop” the crack open (hence they are called “proppant”) and allow the gas from the rock pores to flow past them into the wellbore and thence to the surface.


Apart from S2-3, most of the rest of the Yanbei's gas lies in another group of rock layers called S1-H8 which was prehistorically formed from narrow, slow, meandering rivers, whose now-mineralised riverbeds comprise a mix of fine gas-bearing sand and other non-gas bearing rock.These geological factors make it much harder to extract gas from S1-H8 than from the cleaner, bigger sands of S2-3. In S1-H8, firstly, the non-gas bearing rock is indistinguishable from good sand on the seismic images that we use to locate the reservoir. Secondly, the narrow rivers that formed S1-H8, resulted in thinner, and therefore smaller, gas reservoirs. Thirdly, because the original rivers were sluggish, they allowed finer deposits to settle, leading to a less porous reservoir rock. As if that were not enough, inside S1-H8’s smaller rock pores (compared to S2-3), the individual tiny puffs of gas lack the momentum to overcome relatively massive water droplets glued to the rock by surface tension.



左图:Drilling patterns to reach the reservoir units


右图:Micrographs of the different rock qualities


In any case, in 2014 at the start of the development drilling we, perhaps optimistically, tried the S2-3 frac design on some S1-H8 wells. With hindsight, the results, not surprisingly, were poor. Our investigation concluded that the high-water content slurry was flooding already-wetted rock pores and obstructing gas outflow.


After these initial results, SCP appointed a research taskforce to work on the S1-H8 problem in parallel with the main S2-3 development focus. The taskforce team surveyed the global state of the art for stimulating such reservoirs and concluded that there was no off-the-shelf solution to unlock this gas, and while many companies had tried elsewhere, the one-off success cases could not be directly applied to Yanbei.


Consequently, the taskforce chose a step by step approach the progress into the unknown – choosing as its breakthrough attempt in 2017 a so-called “volume frac” design: It was hoped that by creating much bigger fractures they would propagate far enough to dissipate near-wellbore water blockage effects. Using water was a cost-effective way to increase the size of the fracture. However, the hopes for a breakthrough were dashed as severe water blockage re-occurred, and gas productivity remained low. Nevertheless, this was an important final attempt with water because the next logical step, moving away for water as the main fluid, would entail new challenges and costs.


In 2018, the taskforce’s post-trial review confirmed that the only way to achieve adequate gas flow rates was to reduce the slurry’s water content by adding a non-aqueous fluid. CO2 was selected because even though it is normally a gas, when pressurised it liquefies and so can be easily pumped. However, using 100% CO2 at -40C would have placed excessive thermal stresses on the equipment and would have been very expensive and logistically challenging on our small remote well pads. So, we settled for a 60/40 CO2/water mix instead.

2018年,攻关小组的试后评估证实,要获得足够的天然气流速,唯一的办法是使用CO2这种非水性流体来降低泥浆的含水量。之所以选择二氧化碳,是因为尽管它通常是气体,但加压后会液化,因此很容易泵送。 但使用零下40℃ 的 100% 纯 CO2 会给设备带来过大的热应力,并且对于我们小型偏远的井场而言,成本太高,后勤支持也会面临挑战。 因此,我们把二氧化碳/水的比率设定为60/40 。

图:Liquid carbon-dioxide frac operations


Happily, initial gas rates from this trial were many times higher than previous trials. It was a real step in the right direction to defeat our water block demon. However, the higher rates could not be sustained as long as required. So, this presented us with the next challenge - ie how to scale up the technique to achieve both high production rates and high ultimate recovery volumes.

欣喜的是,这次试验的初始气量比前几次试验高很多倍,标志着我们在打败水锁恶魔的正确方向上迈进了真正一步。 但高产量不能持续所需的时长。 因此,这给我们带来了下一个挑战——如何扩大技术规模,来实现高产量和最终的高采收率?

This brings us to the present day. A lot has happened in the interim in SCP. After successfully completing the technologically demanding execution and start up phases of the project, Schlumberger and Copower agreed to swap roles within their close partnership to give the lead to Copower’s entrepreneurial mindset, underpinned by Schlumberger’s world-class technology.

到目前为止, SCP 做了很多优化调整。成功完成延北项目高技术的建设与投产后,斯伦贝谢与长和实业同意,在其紧密的合作伙伴关系中互换角色,由长和实业来主导实施企业战略,由斯伦贝谢来提供世界一流的技术支持。

Now a reconstituted S1-H8 research taskforce, is harnessing the best of Schlumberger technology and domestic experience to make the next breakthrough. Experience in other basins has shown that multi-organisational collaboration is a must for breakthrough projects like this, and so we are also reaching out to research institutes and other companies.

现在,SCP重新组建了一支山1盒8 攻关小组,该小组正在充分利用斯伦贝谢技术和国内先进经验,来努力取得下一个突破。其他盆地的经验表明,多组织合作是此类项目取得突破的必要条件,因此我们正在与其他研究机构和企业接洽。

Currently the team has identified three focus areas: (i) productivity (ii) cost (iii) well targeting. The top S1-H8 priorities in 2021 remain (i) & (ii) - productivity and cost. Productivity is key because unless a well exceeds a critical gas rate, it will always need expensive nursing to avoid choking on its own water production. For the latest batch of trials we are applying lessons from the CO2 trials to minimise water content, and lessons from recent S2-3 well trials to maximise fracture size.

目前,攻关小组确定了三个重点领域:(i) 产量 (ii) 成本 (iii) 井位。 2021 年,山1盒8攻关的重点仍然是 (i) 和 (ii) ——产量和成本。产量是关键,因为除非气井产量超过了临界产量,否则一直需要采取昂贵的措施,来避免气井因自身出水而导致水堵。在当前批次的试验中,我们吸取了 CO2 压裂试验的经验,把含水量降至最低,也吸取了最近山S2-3试验的经验,使压裂规模最大化。


Cost is also critical to ensure the well is commercial and economic. So, to drive costs down we are also switching from CO2 to a cheaper and more environmentally friendly, nitrogen-based, design.


The third research focus (“well targeting”) aims to push the boundaries of geophysical technology (eg seismic imaging) to accurately locate “sweetspots” – ie areas of bigger and better reservoir rock.


左图:Identifying sweetspots using combined geophysical techniques


右图:Petrophysical logs of the rock properties

From now until 2022, we will write the next exciting chapter of this adventure story to break through the “S1-H8-barrier” and permit large-scale commercial development. There is risk for sure, but also a huge potential prize. We are confident that if it can be done, then SCP, Yanchang, Schlumberger and our committed supporters have the can-do mentality to do it. Stay tuned.



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