Lab to Field Scale Modeling of Low Temperature Air Injection with Hydrocarbon Solvents for Heavy-Oil Recovery in Naturally Fractured Reservoirs

本文研究了在天然裂缝性油藏（NFR）中，利用低温氧化（LTO）条件下的空气注入与烃类溶剂联合回收稠油的方法。由于在蒸汽注入时存在过量的热损失，对于深部NFR中的稠油，增强油回收（EOR）的选择有限。高温氧化（HTO）条件下的空气注入，即原位燃烧（ISC），被认为是水基热应用的替代方案。然而，ISC的实施存在严重限制，包括注入空气的区域分布不佳和燃烧效率低下，以及由于油藏的异质性导致未消耗的注入氧气（O2）可能到达生产井的安全风险。
为了利用空气的低成本和可用性，提出了一种在低温条件下（LTO）使用空气作为增压剂的方法。在这种条件下，会生成含氧化合物，导致油粘度增加，降低流体流动性。为了最小化这种不利影响，可以应用空气注入与烃类溶剂的联合方法。本研究的目标是评估在NFR中含稠油的低温空气注入（LTO）作为一种提高油回收率的方法，阐明烃类溶剂添加到空气中的油回收和O2消耗的影响，并提出最佳条件（温度、空气/溶剂比）和实施策略，以有效使用这种方法。
通过全面的实验室和数值模拟研究来实现这些目标。静态扩散实验通过将含油饱和岩心浸泡在代表基质/裂缝系统的气体中，模拟循环气体注入（huff-and-puff）。油回收和O2消耗是主要评估参数，并研究了包括岩石类型、温度、裂缝体积、溶剂类型、基质尺寸、气体注入序列和浸泡时间在内的一系列变量。从实验研究中得出以下结论：
  1. 气体序列设计影响油回收。
  2. 在空气循环中，浸泡在丁烷而非丙烷后的O2消耗更高。
  3. 与纯空气或溶剂相比，浸泡在空气+C3混合物中的岩心更有益；即，产生的气体中O2浓度更低，溶剂使用量更少，油回收速度更快且更高。
然后，创建了岩心尺度的数值模拟模型，用于模拟实验室实验，并对空气/C3比和基质尺寸进行敏感性分析。结果表明，该过程对基质尺寸极为敏感，基于最小化给定基质尺寸的烃类溶剂，可以实现空气注入（由烃类溶剂辅助）的优化。此外，使用放大到现场尺度的数值模型进行敏感性分析，观察到在被气体填充的裂缝包围的基质块中起作用的油生产机制主要是气体-油重力排驱、有效扩散和气体替代油的空隙。
最后，创建了一个假设NFR的数值模拟扇区模型，并分析了几种空气/气体注入序列。得出结论，低温条件下的空气（LTO）和丙烷注入代表了从NFR中回收稠油的现场尺度替代方案，并且可以为给定的气体注入序列（气体类型和注入/浸泡持续时间）、温度和块尺寸获得最佳的生产时间/浸泡时间比。

CMG软件的应用情况

在本研究中，CMG软件被广泛用于数值模拟研究。具体来说，CMG WINPROP软件包用于生成PVT模型，而CMG STARS软件包用于进行数值模拟。这些软件工具在岩心尺度和现场尺度的模拟中都发挥了关键作用，帮助研究人员分析了空气/C3比和基质尺寸对油回收的影响，并进行了敏感性分析。此外，CMG软件还用于创建假设NFR的数值模拟扇区模型，以分析不同的空气/气体注入序列，并确定最佳的生产时间/浸泡时间比。

作者单位

Jose Ramon Mayorquin-Ruiz 加拿大阿尔伯塔大学土木与环境工程系

Abstract
Alternatives for enhanced oil recovery processes in heavy oil containing deep naturally fractured reservoirs (NFR) are limited due to excessive heat losses when steam is injected. Air injection at high temperature oxidation conditions (in-situ combustion) has been considered as an alternative to aqueous based thermal applications. However, its implementation has serious limitations including poor areal distribution of injected air and poor combustion efficiency due to the heterogeneous nature of these reservoirs as well as the safety risk of unconsumed injected oxygen (O2) reaching the production wells.

Taking advantage of the low cost and availability of air, one option is to use air at low temperature conditions (low temperature oxidation, LTO) as a pressurizing agent in NFR. Oxygenated compounds are generated at these conditions resulting in oil viscosity increase, reducing fluid mobility. In order to minimize this detrimental effect, a combination of air injection with hydrocarbon solvents can be applied. The objectives of this thesis are to evaluate air injection at LTO conditions in NFR containing heavy oil as a way to improve oil recovery, to clarify the effect of hydrocarbon solvent addition into air on oil recovery and O2 consumption, and to propose optimal conditions (temperature, air/solvent ratio) and implementation strategies for an efficient use of this suggested method.

Comprehensive laboratory and numerical simulation studies were conducted to achieve these objectives. Static diffusion experiments—simulating cyclic gas injection (huff-and-puff)—were carried out by soaking heavy oil saturated cores into a reactor filled with gas representing a matrix/fracture system. Oil recovery and O2 consumption were the main parameters assessed and an extensive set of variables including rock type, temperature, fracture volume, solvent type, matrix size, gas injection sequences, and soaking times were studied. From experimental studies, the following conclusions were made:
1. Gas sequence design affects oil recovery,
2. O2 consumption in air cycles is higher after the core is soaked into butane rather than propane,
3. It is beneficial to soak cores in air+C3 mixture rather than pure air or solvent; i.e.,
lower O2 concentration in produced gas, less solvent usage, higher and faster oil recovery compared to alternate injection of air and C3.

Then, core scale numerical simulation models were created for modeling lab experiments for a sensitivity analysis on Air/C3 ratio and matrix size. The results show that the process is extremely sensitive to matrix size and optimization of air injection (assisted by hydrocarbon solvents) can be achieved based on the minimized hydrocarbon solvent for a given matrix size. Additionally, a sensitivity analysis was performed using an up-scaled numerical model to the field scale containing meter-scale matrix blocks. It was observed that oil production mechanisms acting in a matrix block surrounded by gas filling the fractures are predominantly gas-oil gravity drainage, effective diffusion, and voidage replacement of oil by gas.

Finally, a numerical simulation sector model of a hypothetical NFR was created and several air gas injection sequences were analyzed. It was concluded that injection of air (LTO conditions) and propane represents an alternative for heavy oil recovery from NFRs at the field scale, and an optimum production time/soaking time ratio can be obtained for given gas injection sequences (type of gas and injection/soaking durations), temperature, and block sizes.

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